Beamforming device for antenna arrays

ABSTRACT

The improved beamforming devices for communication systems operating in the mm-wave spectrum are particularly designed for antenna architectures consisting of antenna arrays, comprising multiple antenna array elements. The disclosed approaches comprise intelligent two stage searches, wherein information from the first stage is used in the second stage. This significantly reduces the computational complexity compared to the known approaches, with minimal loss in performance.

The present application claims priority to European Patent Application16159898.2 filed by the European Patent Office on 11 Mar. 2016 and16188104.0 on 9 Sep. 2016 and 16197221.1 on 4 Nov. 2016, the entirecontents of which being incorporated herein by reference.

BACKGROUND Field of the Disclosure

The present disclosure relates to a beamforming device and a beamformingmethod for use in a wireless communication system comprising aninitiator and a responder or, more generally, two communication devicesto enable communication with each other through the wirelesscommunication system. Further, the present disclosure relates to acommunication device and a communication system.

Description of Related Art

A significant challenge for the operation in mm-wave bands (i.e.,operating around and/or above 30 GHz) is the strong pathloss,experienced at high frequencies. This can be compensated by the use ofhighly directive antennas composed of many antenna elements, whichde-phase and thereby amplify the transmitted and received signals toobtain desired beamforming effects. Since individually shaping thesignal on each individual antenna element implies the use of a dedicatedRF chain, which is practically cost-ineffective, hybrid architecturesconsisting of multiple antenna arrays (i.e. antennas each havingmultiple antenna elements) have been proposed, e.g. in C. Cordeiro etal, “Next Generation 802.11ad 30+ Gbps WLAN”, IEEE 802.11, May 2014. Inthese “hybrid MIMO settings”, the antenna arrays comprise multiple phaseshifters and are able to perform a coarse analog beamforming, by meansof which the pathloss can be mitigated and thereby communication betweendevices can be enabled.

Analog beamforming corresponds to the act of physically steering one ormore directional beams into a preferred direction, e.g. by means ofanalog phase shifters or by changing the phase characteristics of anantenna array. Further, the complete arrays, rather than each individualelement thereof, are connected to RF chains. Finer digital beamformerscan be created on top of the analog ones. In this manner a full MIMOcapability can be obtained, in which multiple streams can besimultaneously transmitted and spatial multiplexing can be achieved.Digital beamforming corresponds to a more general concept, in which bothamplitudes and phases can be controlled of each transmitted beam. Afterpre-coding at transmitter side, and decoding at receiver side, the beamscan be separated again.

The beamforming design in the mm-wave domain is essentially differentthan the traditional beamforming problems. This is because intraditional beamforming problems, channel state information (CSI) isassumed to be available and beamformers are designed to optimize a givenmetric (e.g., throughput), given the CSI. In the mm-wave domain,obtaining the CSI before the analog beamforming procedure is impossibledue to the weak channels. In the absence of CSI, the optimal analogbeamforming solution can be obtained as the solution of an exhaustivesearch procedure which is computationally very expensive and thusimpractical.

Hereinafter, a single user scenario is considered, in which two devices,further referred to as initiator (or first transceiver or firstcommunication device, e.g., access point or station) and responder (orsecond transceiver or second communication device, e.g., station oraccess point), each equipped with one or more antenna arrays, includingone or more antenna array elements), aim to find the optimal beamformers(i.e. the optimal antenna beam combination), in the sense that maximumthroughput can be achieved between them.

For this problem, a solution has been proposed in the 802.11 ad standard(IEEE Working Group, “Wireless LAN Medium Access (MAC) and PhysicalLayer (PHY) Specification Amendment 3: Enhancements for Very HighThroughput in the 60 GHz Band,” 2012), under the simplifying assumptionthat only one antenna array is present or actively used at bothinitiator and responder thus supporting only single input single output(SISO) communication. The method consists of two steps. In the firststep, referred to as sector level sweep (SLS), a sector is swept with adirectional beam at the initiator and a quasi-omnidirectional beam atthe responder (“listening in all directions”). The strongest sectors(allowing maximum throughput and/or received signal strength) are fedback from responder to initiator. The procedure is reversed and repeatedfor the responder. The beams obtained in this manner are further refinedin a second stage, referred to as beam refinement phase (BRP), wherecombinations of beams are tested in order to obtain the strongestchannel.

Further, in H. Persson, “Efficient Beam Selection for HybridBeamforming”, IEEE 802.11, September 2015 a pairwise search algorithm isdescribed for the general problem in which arbitrary numbers of antennaarrays are considered at both initiator and responder. In the firststage, each pair of antenna arrays at initiator and responder isconsidered separately. The beams that maximize the SISO capacity (ore.g. SNR) between each pair are found, using an exhaustive search forall beam directions on both sides. The beams that maximize the SISOcapacity (i.e. SNR) between each pair are found, using an exhaustivesearch. The pair of antenna arrays at initiator and responder, whichobtained the best SISO capacity and the corresponding beams are thenfixed for the second stage. In this second stage the combinations of twofurther (at least partly not yet fixed) antenna arrays are consideredand multiple input multiple output (MIMO) capacity between them iscomputed for all beam combinations, which are not yet fixed from theprevious stage. This method is still computationally expensive as itconsists of several stages of exhaustive search approaches.

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventor(s), to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentdisclosure.

SUMMARY

It is an object to provide a beamforming device and a beamforming methodfor use in a wireless communication system with reduced computationalcomplexity, reduced time consumption and minimal loss of performancewith respect to known solutions. It is a further object to provide acommunication device, a communication system as well as a correspondingcomputer program for implementing the disclosed beamforming methods anda non-transitory computer-readable recording medium for implementing thedisclosed beamforming methods.

According to an aspect there is provided a beamforming device for use ina wireless communication system, said beamforming device comprising

-   -   a control unit for controlling the initiator having one or more        initiator antenna arrays each comprising two or more initiator        antenna elements and/or the responder having one or more        responder antenna arrays each comprising two or more responder        antenna elements, wherein the initiator has at least two        initiator antenna arrays and/or the responder has at least two        responder antenna arrays,    -   a processing unit for selecting one or more initiator antenna        beams per initiator antenna array and one or more responder        antenna beams per responder antenna array for use by the        initiator and the responder in communicating with each other,    -   wherein said control unit and said processing unit are        configured    -   i) to control, by the control unit in a first training stage,        the initiator antenna elements, per pair of initiator antenna        array and responder antenna array, to transmit a first training        signal by successively using different initiator antenna beams        of different initiator antenna beam directions and to receive a        first training signal transmitted by the responder antenna array        by successively using different responder antenna beams of        different responder antenna beam directions,    -   ii) to select, by the processing unit, a sub-set of the antenna        beam combinations that have been used in the first training        stage or are derived from the antenna beam combination used in        the first training stage, for use in a second training stage,        wherein at least some of the antenna beam combinations of said        sub-set are selected by use of first responder quality        information indicating the quality of reception of the first        training signals by the respective responder antenna array for        the different initiator antenna beams used by the respective        initiator antenna array for transmitting the first training        signal and first initiator quality information indicating the        quality of reception of the first training signals by the        respective initiator antenna array for the different responder        antenna beams used by the respective responder antenna array for        transmitting the first training signal in the first training        stage, wherein the first responder quality information and the        first initiator quality information obtained for the different        pairs of initiator antenna arrays and responder antenna arrays        of the first training stage is used,    -   iii) to control, by the control unit in the second training        stage, the initiator antenna elements of the initiator antenna        arrays to transmit a second training signal by successively        using different initiator antenna beams of different initiator        antenna beam directions according to one or more of the selected        antenna beam combinations, and/or to receive a second training        signal transmitted by the responder antenna elements of the one        or more responder antenna arrays by successively using different        responder antenna beams of different responder antenna beam        directions according to one or more of the selected antenna beam        combinations, and    -   iv) to select, by the processing unit, a final antenna beam        combination for use by the initiator and the responder in        communicating with each other from second responder quality        information indicating the quality of reception of the second        training signals by the responder antenna arrays for the        different initiator antenna beams used by the initiator antenna        arrays for transmitting the second training signal and/or from        second initiator quality information indicating the quality of        reception of the second training signals by the initiator        antenna arrays for the different responder antenna beams used by        the responder antenna arrays for transmitting the second        training signal in the second training stage.

According to another aspect there is provided a beamforming device foruse in a wireless communication system, said beamforming devicecomprising

-   -   a control unit for controlling the initiator having one or more        initiator antenna arrays each comprising two or more initiator        antenna elements and/or the responder having one or more        responder antenna arrays each comprising two or more responder        antenna elements, wherein the initiator has at least two        initiator antenna arrays and/or the responder has at least two        responder antenna arrays,    -   a processing unit for selecting one or more initiator antenna        beams per initiator antenna array and one or more responder        antenna beams per responder antenna array for use by the        initiator and the responder in communicating with each other,

wherein said control unit and said processing unit are configured

i) to control, by the control unit in a first training stage, theresponder antenna elements, per pair of initiator antenna array andresponder antenna array to receive a first training signal transmittedby the initiator antenna array by successively using different initiatorantenna beams of different initiator antenna beam directions and totransmit a first training signal by successively using differentresponder antenna beams of different responder antenna beam directions,

ii) to select, by the processing unit, a sub-set of the antenna beamcombinations that have been used in the first training stage or arederived from the antenna beam combination used in the first trainingstage, for use in a second training stage, wherein at least some of theantenna beam combinations of said sub-set are selected by use of firstresponder quality information indicating the quality of reception of thefirst training signals by the respective responder antenna array for thedifferent initiator antenna beams used by the respective initiatorantenna array for transmitting the first training signal and firstinitiator quality information indicating the quality of reception of thefirst training signals by the respective initiator antenna array for thedifferent responder antenna beams used by the respective responderantenna array for transmitting the first training signal in the firsttraining stage, wherein the first responder quality information and thefirst initiator quality information obtained for the different pairs ofinitiator antenna arrays and responder antenna arrays of the firsttraining stage is used,

iii) to control, by the control unit in the second training stage, theresponder antenna elements of the responder antenna arrays to receive asecond training signal transmitted by the initiator antenna elements ofthe one or more initiator antenna arrays by successively using differentinitiator antenna beams of different initiator antenna beam directionsaccording to one or more of the selected antenna beam combinationsand/or to transmit a second training signal by successively usingdifferent initiator antenna beams of different initiator antenna beamdirections according to one or more of the selected antenna beamcombinations, and

iv) to select, by the processing unit, a final antenna beam combinationfor use by the initiator and the responder in communicating with eachother from second responder quality information indicating the qualityof reception of the second training signals by the responder antennaarrays for the different initiator antenna beams used by the initiatorantenna arrays for transmitting the second training signal and/or fromsecond initiator quality information indicating the quality of receptionof the second training signals by the initiator antenna arrays for thedifferent responder antenna beams used by the responder antenna arraysfor transmitting the second training signal in the second trainingstage.

According to a further aspect there is provided a communication devicefor communicating with another communication device in a wirelesscommunication system, said communication device comprising one or moreantenna arrays each comprising two or more antenna elements, and abeamforming device as disclosed herein.

According to a further aspect there is provided a communication systemcomprising a beamforming device as disclosed herein and two or morecommunication devices, each having at least one antenna array eachcomprising two or more antenna elements, wherein at least onecommunication device has at least two antenna arrays.

According to still further aspects a computer program comprising programmeans for causing a computer to carry out the steps of the methoddisclosed herein, when said computer program is carried out on acomputer, as well as a non-transitory computer-readable recording mediumthat stores therein a computer program product, which, when executed bya processor, causes the method disclosed herein to be performed areprovided.

Embodiments are defined in the dependent claims. It shall be understoodthat the disclosed methods, the disclosed communication device, thedisclosed communication system, the disclosed computer program and thedisclosed computer-readable recording medium have similar and/oridentical further embodiments as the claimed transmitter and as definedin the dependent claims and disclosed herein.

One of the aspects of the disclosure is to provide improved beamformingtraining solutions for communication systems operating in the mm-wavespectrum. The disclosed approaches are particularly designed for antennaarchitectures consisting of multiple antenna arrays on the transmitterside (initiator side) and the receiver side (responder side), eachantenna array comprising two or more antenna elements), which are seenas cost-effective enablers for communication in high frequency bands.Further, the disclosed approaches can generally also be used in SIMO orMISO configurations, in which the initiator or the responder comprises asingle antenna array only. The disclosed solutions comprise intelligenttwo stage searches, wherein information from the first stage is used inthe second stage. This significantly reduces the computationalcomplexity compared to the known approaches, with minimal loss inperformance or even better performance. Hence, with the disclosedsolutions one or more initiator antenna beams per initiator antennaarray and one or more responder antenna beams per responder antennaarray are finally found for use by the initiator and the responder incommunicating (uni-directionally or bi-directionally) with each other.

It should be noted that in the context of the present disclosure theterm “successively” shall generally be understood as starting one afteranother or with minimal overlap, but does not necessarily imply an orderof operation.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows a schematic diagram of different embodiments of the generallayout of a communication system according to the present disclosure,

FIG. 2 shows a communication system illustrating a hybrid architectureof the initiator and the responder,

FIG. 3 shows a schematic diagram of a first detailed embodiment of acommunication system according to the present disclosure,

FIG. 4 shows a flow chart of a first embodiment of a beamforming methodaccording to the present disclosure for use in the communication systemshown in FIG. 3,

FIG. 5 shows a schematic diagram of a second detailed embodiment of acommunication system according to the present disclosure,

FIG. 6 shows a flow chart of a second embodiment of a beamforming methodaccording to the present disclosure for use in the communication systemshown in FIG. 5,

FIG. 7 shows a diagram of an embodiment of a communication system forillustrating a genetic search algorithm,

FIGS. 8A and B show diagrams illustrating the use of antenna scores andan overall score for selecting antenna beam combinations,

FIG. 9 shows a schematic diagram of another embodiment of a beamformingdevice according to the present disclosure,

FIGS. 10A and B show diagrams illustrating a known SSW feedback frameand a known SSW feedback field,

FIGS. 11A and B show diagrams illustrating a modified SSW feedback frameand a modified SSW feedback field,

FIGS. 12A and B show diagrams illustrating other embodiments of amodified SSW feedback field,

FIGS. 13 A and C show diagrams illustrating another embodiment of amodified SSW feedback frame and SSW ACK frames,

FIG. 13B shows a diagram of a modified SSW Feedback Field,

FIGS. 14A and B show diagrams illustrating other embodiments of amodified SSW feedback field,

FIGS. 15A to C show diagrams illustrating modified SSW Feedback and ACKframes with variable numbers of sectors,

FIG. 16 shows another embodiment of a short SSW frame including afragmented bit field, and

FIG. 17 shows another embodiment of a short SSW frame including afragmented bit field and a field indicating the number of sectorspresent in an additional frame.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views. FIG. 1shows schematic diagrams of various general embodiments of acommunication system according to the present disclosure. Thecommunication system generally comprises an initiator 10 (e.g. an accesspoint), one or more responders 20 (e.g. stations, such as a smartphone,laptop, etc.) and one or more beamforming devices 30. When the initiator10 and a responder 20 seek to communicate with each other(uni-directionally or bi-directionally) the optimal (in the sense ofthroughput and signal quality) beamforming, i.e. the optimal antennabeam combination of the antenna beam used by the initiator 10 and theresponder 20, shall be found. This process of finding the optimal analogbeamforming with low computational complexity and in a short time isaddressed in this disclosure. In addition to analog beamforming digitalbeamforming can optionally be applied, which is not further addressedherein.

For this purpose the beamforming device 30 can be part of the initiator10 (as in the communication system 1 a shown in FIG. 1A) togetherforming a first communication device 40, can be part of the responder 20(as in the communication system 1 b shown in FIG. 1B) together forming asecond communication device 41, or can be provided as separate entity(as in the communication system 1 c shown in FIG. 1C). Even further, abeamforming device 30 can also be provided in the initiator 10 and eachresponder 20, or the tasks of the beamforming device 30 can be splitbetween different beamforming devices provided in the initiator 10 andeach responder 20.

Before details of the disclosed approaches are explained, someexplanations shall be provided about analog and hybrid beamforming ingeneral. FIG. 2 shows a schematic diagram of a communication system 2comprising an initiator 10 and a responder that wish to communicate overthe channel (described by channel matrix H) to communicate with eachother. The initiator 10 generally has one or more (two in thisembodiment) initiator antenna arrays 11, 12, each comprising two or more(four in this embodiment) initiator antenna elements 110, 120. Theresponder 20 has one or more (two in this embodiment) responder antennaarrays 21, 22 each comprising two or more (four in this embodiment)responder antenna elements 210, 220. Generally, at least one of theinitiator 10 and the responder 20 has at least two antenna arrays sothat single input multiple output (SIMO), multiple input single output(MISO) or MIMO communication scheme can be used for the communication.

Analog beamforming is often implemented using a network of digitallycontrolled phase shifters. In this configuration, the antenna elements110, 120, 210, 220 belonging to one antenna array 11, 12, 21, 22 areconnected via phase shifters 111, 121, 211, 221 to a single RF chain 13,14, 23, 24, as illustrated in FIG. 2 showing a communication system 2using a hybrid architecture of the initiator 10 and the responder 20.Generally, the phase shifter weights are adaptively adjusted usingdigital signal processing using a specific strategy to steer one or morebeams and meet a given objective, for example to maximize receivedsignal power. The hybrid architecture shown in FIG. 2 uses MIMOcommunication at mm-wave frequencies and comprises, in addition to theanalog domain, a digital domain. In the digital domain basebandprecoding and combining is performed using a baseband (BB) processingcircuit 15, 25 coupled to the respective RF chains 13, 14, 23, 24. Moredetails of such a hybrid architecture as well as the function and theprocessing can be found in the documents cited in the backgroundsection.

FIG. 3 shows a schematic diagram of an embodiment of a communicationsystem 3 according to the present disclosure. In this embodiment theinitiator 10 comprises the beamforming device 30 a, as shown in FIG. 1A.The beamforming device 30 a generally comprises a control unit 31 forcontrolling the initiator 10 and a processing unit 32 for selecting oneor more initiator antenna beams per initiator antenna array 11, 12 andone or more responder antenna beams per responder antenna array 21, 22for use by the initiator 10 and the responder 20 in communicating witheach other.

The method performed by the beamforming device generally comprises twostages, a first training stage and a second training stage. In the firsttraining stage a SISO-like training is performed between different pairsof antenna arrays (i.e. between an initiator antenna array 11, 12 of theinitiator 10 and a responder antenna array 21, 22 of the responder 20)to pre-select antenna combinations. The pre-selected antennacombinations are then further tested and/or used as starting points forfurther testing in the second training stage, in which a MIMO-, SIMO-and/MISO-like training is performed between one or more initiatorantenna arrays 11, 12 and one or more responder antenna arrays 21, 22 ofthe responder 20 to find the best antenna beam combination for use inthe subsequent combination of antenna beams, i.e. which antenna beamshall be used by each of the antenna arrays 11, 12, 21, 22. On top ofthis two-stage analog beamforming procedure, additional digitalbeamforming may be performed.

FIG. 4 shows a flow chart of an embodiment of a beamforming method 300according to the present disclosure that may be carried out by thebeamforming device 30 a in the scenario illustrated in FIG. 3.

In a first step 301, referred to as first training stage, the controlunit 31 controls the initiator antenna elements 110, 120, per pair ofinitiator antenna array 11, 12 and responder antenna array 21, 22, totransmit (in a first phase per pair) a first training signal bysuccessively using different initiator antenna beams of differentinitiator antenna beam directions and to receive (in a second phase perpair) a first training signal transmitted by the responder antenna array21, 22 by successively using different responder antenna beams ofdifferent responder antenna beam directions. The different responderantenna beams successively used by the responder antenna arrays 21, 22may also be controlled by the control unit 31 of the beamforming device30 a (e.g. through control information transmitted to the responder 20),or may be controlled by a control unit of a separate beamforming deviceprovided in the responder 20, or may be prescribed in advance (i.e. ifthe first training stage is started, a certain prescribed procedure iscarried out).

In a second step 302 the processing unit 32 selects a sub-set of theantenna beam combinations that have been used in the first trainingstage for use in a second training stage. Hereby, at least some of theantenna beam combinations of said sub-set are selected by use of firstresponder quality information (e.g. a responder antenna score)indicating the quality of reception of the first training signals by therespective responder antenna array for the different initiator antennabeams used by the respective initiator antenna array for transmittingthe first training signal and first initiator quality information (e.g.an initiator antenna score) indicating the quality of reception of thefirst training signals by the respective initiator antenna array for thedifferent responder antenna beams used by the respective responderantenna array for transmitting the first training signal in the firsttraining stage. Hereby, the first responder quality information and thefirst initiator quality information obtained for the different pairs ofinitiator antenna arrays and responder antenna arrays of the firsttraining stage is used.

In an implementation, the first responder quality information may beprovided by the responder 20 to the initiator 10, e.g. by transmissionas separate data or by transmission along with or included in the firsttraining signals transmitted by the responder antenna arrays in thesecond phase of the first training stage, and the first initiatorquality information may be determined or calculated by the initiator 10itself, e.g. by the processing unit 32.

The first initiator quality information and the first responder qualityinformation is generally information indicating signal to noise ratio,signal to noise-and-interference ratio, received signal strengthindication, the estimated capacity, received electric or magnetic fieldstrength or delay spread per pair of initiator antenna array 11, 12 andresponder antenna array 21, 22 and per antenna beam (i.e. per initiatorantenna beam and per responder antenna beam).

In a third step 303, referred to as second training stage, the controlunit 31 controls the initiator antenna elements 110, 120 of theinitiator antenna arrays 11, 12 to commonly (i.e. simultaneously orsuccessively), transmit a second training signal by successively usingdifferent initiator antenna beams of different initiator antenna beamdirections according to the selected antenna beam combinations and/or tocommonly receive with initiator antenna beams set according to theantenna beam combinations, selected in the first stage, a secondtraining signal commonly transmitted by the responder antenna elements210, 220 of the one or more responder antenna arrays 21, 22 bysuccessively using different responder antenna beams of differentresponder antenna beam directions according to the selected antenna beamcombinations.

In a fourth step 304 the processing unit selects a final antenna beamcombination for use by the initiator 10 and the responder 20 incommunicating with each other from second responder quality informationindicating the quality of reception of the second training signals bythe responder antenna arrays for the different initiator antenna beamsused by the initiator antenna arrays for transmitting the secondtraining signal and/or from second initiator quality informationindicating the quality of reception of the second training signals bythe initiator antenna arrays for the different responder antenna beamsused by the responder antenna arrays for transmitting the secondtraining signal in the second training stage.

In an implementation, the second responder quality information may beprovided by the responder 20 to the initiator 10, e.g. by transmissionas separate data or by transmission along with or included in the secondtraining signals transmitted by the responder antenna arrays, and thesecond initiator quality information may be determined by the initiator10 itself, e.g. by the processing unit 32.

In an implementation, step 303 may comprise several sub-phases, wherein,in a first sub-phase a sub-set of the beam combinations, selected in thefirst training stage, are tested as described in step 303, and secondresponder quality information is computed. Depending on the secondresponder quality information, a second sub-phase can be performed as instep 303, wherein a different sub-set of the beam combinations, computedin the first training stage is used and second responder qualityinformation is computed. This can be repeated until a terminationcriterion is met.

The second initiator quality information and/or the second responderquality information is information indicating the quality of theresulting communication link, such as the estimated capacity, sum ofsingular values, condition number, signal to interference plus noiseratio, or signal power of a channel matrix per complete antenna beamcombination.

Hence, in the first training stage a coarse beamforming solution isfound. In a particular implementation the procedure may (but needs notnecessarily) be similar to the sector level sweep (SLS), currentlyincluded in the 802.11 lad standard: In a first phase of the firsttraining stage the initiator 10 forms a directional beam with the firstantenna array 11 and sweeps a sector, while the responder 20 listenswith the first antenna array 21 set to a quasi-omnidirectional pattern.The same is performed with the directional beam formed by the secondantenna array 12 at the initiator 10, while the responder 20 listenswith second antenna array 22 set to quasi-omnidirectional pattern, thenwith the third antenna array and so on (most use-cases consider only twoantenna arrays on each side, i.e., 2×2 case). The procedure is thenrepeated in a reversed fashion in a second phase of the first trainingstage, with the responder 20 creating the directional beams and theinitiator 10 listening with quasi-omnidirectional patterns.

Hence, in this implementation the control unit 31 controls, in step 301,the initiator antenna elements 110, 120, per pair of initiator antennaarray 11, 12 and responder antenna array 21, 22, to transmit (in a firstphase) the first training signal by successively using differentinitiator antenna beams of different beam directions to a responderantenna array configured to receive the first training signal with anomnidirectional or wide-angle responder antenna beam and to receive (ina second phase) a first training signal transmitted by a responderantenna array 21, 22 by successively using different responder antennabeams of different beam directions, wherein the initiator antenna array11, 12 is configured to receive the first training signal with anomnidirectional or wide-angle initiator antenna beam. In an alternativeimplementation, in the second phase, the initiator antenna elements 110,120 may be controlled to transmit a first training signal with an (e.g.omnidirectional or wide-angle initiator antenna beam, wherein theresponder antenna elements 210, 220 are controlled to receive thetransmitted first training signal by successively using differentresponder antenna beams of different beam directions.

In another scenario the control unit 31 may control the initiatorantenna arrays to commonly receive a first training signal transmittedby a responder antenna array 21, 22 by successively using differentresponder antenna beams of different beam directions, wherein theinitiator antenna arrays 11, 12 are configured to receive the firsttraining signal with an omnidirectional or wide-angle initiator antennabeam. The initiator antenna array, which attains the best receptionquality for the antenna beams, transmitted by a responder antenna array21, 22, will be considered paired to this responder antenna array andthe scores for the second training stage will be defined per this pair.

In another implementation the control unit 31 controls, in step 301, theinitiator antenna elements 110, 120, per pair of initiator antenna array11, 12 and responder antenna array 21, 22, to transmit (in a firstphase) the first training signal by successively using differentinitiator antenna beams of different initiator antenna beam directionsto a responder antenna configured to receive the first training signalwith a directed responder antenna beam, wherein the successivetransmission of the first training signal with different initiatorantenna beams of different initiator antenna beam directions is repeatedmultiple times, wherein in each iteration one or more different directedresponder antenna beams are used for reception, and to receive (in asecond phase) the first training signal transmitted from a responderantenna array 21, 22 by successively using different responder antennabeams of different responder antenna beam directions, wherein theinitiator antenna array 11, 12 is configured to receive the firsttraining signal with a directed initiator antenna beam, wherein thesuccessive transmission of the first training signal with differentresponder antenna beams of different responder antenna beam directionsis repeated multiple times, wherein in each iteration one or moredifferent directed initiator antenna beams are used for reception. Thus,in this implementation a full or partial exhaustive or a pairwise searchmay be performed.

The training in the first training stage for all pairs of antenna arrays11+21, 12+22, etc. between initiator 10 and responder 20 can beperformed sequentially (more time consuming) or in parallel. Paralleltraining requires suppression of cross-talk. In a 2×2 case, where thetwo antenna arrays on each side apply different polarization (e.g.,horizontal and vertical, left-hand circular and right-hand circular(i.e. both polarizations are used, but rotating in differentdirections)), the channel itself may provide sufficient suppression. Inother cases, or in addition to channel-induced cross-talk attenuation,different (quasi-)orthogonal training sequences (waveforms) may be usedon each antenna array. Correlating with the corresponding sequences atreceiver side may suppress the influence from undesired trainingsequences. During this first stage information on quality (e.g. SNR) ofdifferent beams is stored for use within the second training stage.

In still another implementation the control unit 31 controls, in a firstphase of the first training stage (i.e. step 301) and/or a secondtraining stage (i.e. step 303), the initiator antenna elements 110, 120to transmit the respective (first or second) training signal bysuccessively using different initiator antenna beams having a first beamwidth and/or the responder antenna elements 210, 220 to receive therespective (first or second) training signal by successively usingdifferent responder antenna beams having a first beam width. Theprocessing unit 32 then selects at least part of the antenna beamcombinations of the sub-set of antenna beam combinations for use in asecond phase of the same training stage based on the initiator antennabeam direction of the initiator antenna beam selected based on theresponder quality information, e.g. providing the best responder qualityinformation.

Further, the control unit 31 controls, in the second phase of the sametraining stage, the initiator antenna elements 110, 120 to transmit thetraining signal by successively using different initiator antenna beamshaving a second beam width different from the first beam width used inthe first phase and having an initiator antenna beam direction identicalor similar as the initiator antenna beam direction (in particular closeto or within the initiator antenna beam sector) of the initiator antennabeam providing the best first initiator quality information in the firstphase and/or the responder antenna elements 210, 220 to receive thetraining signal by successively using different responder antenna beamshaving a second beam width different from the first beam width used inthe first phase and having a responder antenna beam direction identicalor similar as the responder antenna beam direction of the responderantenna beam providing the best first responder quality information inthe first phase.

Thus, according to this implementation variable beam (sector) widths maybe used in the different phases. For instance, it is started with largerbeam widths to find the best sectors, and then these best sectors arefurther investigated using smaller beam widths.

FIG. 5 shows a schematic diagram of another embodiment of acommunication system 4 according to the present disclosure. In thisembodiment the responder 20 comprises the beamforming device 30 b, asshown in FIG. 1B. Also in this embodiment the beamforming device 30 bgenerally comprises a control unit 31 for controlling the responder 20and a processing unit 32 for selecting one or more responder antennabeams per responder antenna array 21, 22 and one or more transmitterantenna beams per transmitter antenna array 11, 12 for use by theinitiator 10 and the responder 20 in communicating with each other.

FIG. 6 shows a flow chart of an embodiment of a beamforming method 400according to the present disclosure that may be carried out by thebeamforming device 30 b in the scenario illustrated in FIG. 5.

In step 401 the control unit 31 controls, in a first training stage, theresponder antenna elements 210, 220, per pair of initiator antenna array11, 12 and responder antenna array 21, 22 to receive a first trainingsignal transmitted by the initiator antenna array 11, 12 by successivelyusing different initiator antenna beams of different initiator antennabeam directions and to transmit a first training signal by successivelyusing different responder antenna beams of different responder antennabeam directions.

In second step 402 the processing unit 32 selects a sub-set of theantenna beam combinations that have been used in the first trainingstage for use in a second training stage, wherein at least some of theantenna beam combinations of said sub-set are selected by use of firstresponder quality information indicating the quality of reception of thefirst training signals by the respective responder antenna array for thedifferent initiator antenna beams used by the respective initiatorantenna array for transmitting the first training signal and firstinitiator quality information indicating the quality of reception of thefirst training signals by the respective initiator antenna array for thedifferent responder antenna beams used by the respective responderantenna array for transmitting the first training signal in the firsttraining stage.

In a third step 403 the control unit 31 controls, in the second trainingstage, the responder antenna elements 210, 220 of the responder antennaarrays 21, 22 to receive a second training signal commonly transmittedby the initiator antenna elements 110, 120 of the one or more initiatorantenna arrays 11, 12 by successively using different initiator antennabeams of different initiator antenna beam directions according to one ormore of the selected antenna beam combinations and/or to commonlytransmit a second training signal by successively using differentinitiator antenna beams of different initiator antenna beam directionsaccording to one or more of the selected antenna beam combinations.

In another scenario the responder may commonly receive with multipleantenna arrays, set according to the beam combinations selected in thefirst training stage, the second training signal transmitted by aninitiator antenna by commonly transmitting using different initiatorantenna beams of different initiator antenna beam directions accordingto one or more of the selected antenna beam combinations.

In a fourth step 404 the processing unit 32 selects a final antenna beamcombination for use by the initiator 10 and the responder 20 incommunicating with each other from second responder quality informationindicating the quality of reception of the second training signals bythe responder antenna arrays for the different initiator antenna beamsused by the initiator antenna arrays for transmitting the secondtraining signal and/or from second initiator quality informationindicating the quality of reception of the second training signals bythe initiator antenna arrays for the different responder antenna beamsused by the responder antenna arrays for transmitting the secondtraining signal in the second training stage.

In an implementation, the second initiator quality information may beprovided by the initiator 10 to the responder 20, e.g. by transmissionas separate data or by transmission along with second training signalstransmitted by the initiator antenna arrays, and the second responderquality information may be determined by the responder 20 itself, e.g.by the processing unit 32.

Also the beamforming device 30 b can be configured further in a similaror equivalent manner as explained above for the various implementationsof the beamforming device 30 a.

In another embodiment the processing unit 32 is configured to select,per pair of initiator antenna array 11, 12 and responder antenna array21, 22, combinations of initiator antenna beams and responder antennabeams to be used in the first training stage by use of a genetic orevolutionary search algorithm. Further, the processing unit 32 may beconfigured to select the sub-set of antenna beam combinations for use inthe second training stage by use of a genetic or evolutionary searchalgorithm. Hereby, part of the antenna beam combinations of the sub-setfor use in the second training stage may be selected randomly, inparticular by use of a uniform or non-uniform probability distribution.Further, in each iteration of step iv) in the second training stage, thesecond responder quality information and/or the second initiator qualityinformation may be used to determine an overall score for the overallantenna beam combination used in said iteration and to compare thedetermined overall score with the overall score of the previousiterations. Hence, in each iteration the overall antenna beamcombination having the best overall score up to said iteration, may beset as preliminary best antenna beam combination and the overall antennabeam combination to be used in the next iteration may be selected basedon the preliminary best antenna beam combination.

Thus, the coarse beams obtained in the first stage may both be utilizedas initial beams for a genetic search algorithm, in which combinationsof beams are tested. The genetic search comprises testing the bestobtained beam with crossover variants i.e., combinations of the bestobtained beams at the initiator and random beams at the responder andvice versa. More precisely, during each iteration of the genetic searchalgorithm, the following combinations may be tested: 1) random beams forthe initiator antenna arrays and the best beams at the responder, 2)random beams for the responder antenna arrays and the best beams at theinitiator, 3) random beams for both initiator and responder, and 4) bestobtained beams. The random beams are generated such that the beams whichhave obtained a high score in the first training phase are more likelyto be chosen. In general, all possible permutations or a subset of arandom guess and the best obtained beams may be possible as well.

An exemplary embodiment of the genetic search algorithm that may be usedaccording to the present disclosure will be illustrated with referenceto FIG. 7 showing a 2×2 configuration of a transmitter Tx (e.g.representing the initiator) having two antenna arrays Tx1, Tx2 eachincluding two transmit antenna elements (not shown) each providing aseparate transmit antenna beam 51, 52, 61, 62 and a receiver Rx (e.g.representing the responder) having two receive antenna arrays Rx1, Rx2each including two antenna elements (not shown) each providing aseparate receive antenna beam 71, 72, 81, 82.

In more detail, i=(i₁,i₂) defines a beam for each transmit antenna suchthat i₁∈{51, 52} and i₂∈{61, 62} and j=(j₁,j₂) defines a beam for eachreceive antenna such that j₁∈{71, 72} and j₂∈{81, 82}. At each iterationx, the input is the best combination of beams seen until iteration x−1,e.g. (i^((*)), j^((*)))=(51 61, 71, 81), with its associated best metrice.g. capacity value C(51 61, 71, 81)=8.5.

The execution may be as follows: In a first step beam combination guess(i j)=(52 62, 72 82) is generated according to a certain probabilitydistribution. In a second step crossovering is performed between(i^((*)), j^((*))) and (i, j) to form two new combinations (i^((*)),j)=(51 61, 72 82) and (i,j^((*)))=(52 62, 71 81). In a third stepcalculation of the metric (capacity) associated to the new threecombinations is performed: C(i, j)=C(52 62, 72 82)=8, C(i^((*)),j)=C(5161, 72 82)=8.3, C(i,j^((*)))=C(52 62, 71 81)=8.7. In a fourth step thecombination achieving the highest capacity among (i^((*)), j^((*))) (i,j) (i^((*)),j) (i,j^((*))) is set as new best combination for iterationx+1: (i,j^((*)))=(52 62, 71 81). The output is the best combination ofbeams (i,j^((*)))=(52 62, 71 81) seen until iteration x with itsassociated best capacity value C(52 62, 71 81)=8.7.

The beam combination guess may be made randomly, e.g. generatedaccording to a uniform probability distribution with all combinations ofbeams having the same probability to be selected as guess. The beamcombination guess may also be made with a priori knowledge, e.g.generated according to a non-uniform probability distribution or witheach combination of beams having a certain probability to be selected asguess which is dependent on the score attached to it (i.e., higherscore→higher probability). The crossover operation shown is just onepossibility to generate the crossed combinations to investigate.

Various other metrics than MIMO capacity are possible, including signalto interference noise ratio, condition number, strongest tap, delayspread, rank of channel matrix, signal to noise ratio, etc.

The genetic search algorithm may be applied in the first training stageon top of the SISO training and/or during the second training stage forwhich guesses are either random or exploit SISO scores of the firsttraining stage. In may further be applied in a pairwise searchalgorithm. e.g. in the first training stage of SISO training to replacea brute-force exhaustive search where beam combination guesses arerandom since there is no a priori information or in the second trainingstage of MIMO training to replace brute-force exhaustive search wherebeam combination guesses are random or exploit SISO information score ofthe first training stage.

In another embodiment the processing unit 32 uses the beam direction ofthe initiator antenna beam and/or the responder antenna beam providingthe best initiator quality information and the best responder qualityinformation for selecting antenna beam combinations used subsequently inthe first training stage and/or the second training stage. Hence, in animplementation based on a desired metric, e.g. the signal to noise ratio(SNR) obtained in the first training stage, a score is computed for eachcombination of beams at the initiator 10 and responder 20. This scoremay be defined as the product (or sum) of the individual scores of thecomponents of this combination. The beam combinations which obtained thebest scores in the first training stage are now further tested in thesecond stage (also called beam refinement phase; BRP). The goal of thissecond training stage is to evaluate the MIMO capacity, which can beobtained when each device uses all antenna arrays with directional beamssimultaneously. The reduction in complexity with respect to theexhaustive search is of more than one order of magnitude whereas theachieved MIMO capacity is only approx. 1% from the optimum after teniterations of the second training stage. The scheme can be implementedwith minimal changes to the frame structures, currently considered inthe 802.11 lad BRP stage, allowing significant complexity reductioncompared with known approaches such as pairwise search or astraightforward extension of 802.11ad procedure using exhaustive searchin BRP. Further, there exists an algorithm which can find the K bestcombinations (e.g. products) from a large set of all possiblecombinations. More details of the best K search algorithm shall beprovided in the following.

Let T₁, . . . , T_(N) denote N vectors with measured SNR values (or anyother score, which relates directly or indirectly to the signalquality). Each vector T_(n) consists of L SNR values denoted by t_(n)^((i))∈T_(n), wherein i denotes the index inside this vector, i.e.,T _(n)=[t _(n) ⁽¹⁾ ,t _(n) ⁽²⁾ , . . . ,i _(n) ^((L))].

It is assumed that the values are already ordered, i.e., t_(n)^((i))≥t_(n) ^((i+1))≥t_(n) ^((i+2)) . . . . Given these tables, it islooked for the K best combinations such that the sums

${S^{(k)} = {\underset{{such}\mspace{14mu}{that}\mspace{14mu}{index}\mspace{14mu}{set}\mspace{14mu} I^{(k)}}{\underset{{t_{n}^{(i)} \in T_{n}},}{\sum\limits_{n = 1}^{N}}}t_{n}^{(i)}}},{S^{(1)} \geq S^{(2)} \geq \cdots \geq S^{(K)}}$

are as large as possible. Alternatively, the product of the individualvalues can also be considered. Applying the monotonically logarithmfunction on a product transfers this combination to above sum.

The corresponding combination of vectors indices can be denoted asI ^((k))=[i ₁ ^((k)) , . . . ,i _(N) ^((k))]

where i_(n) ^((k)) gives the index of the element that is chosen fromthe nth vector T_(n) for the kth best combination. It can immediately beseen that the best solution S⁽¹⁾ is given by picking the elements withthe maximum value for each vector T_(n). Instead of using a brute forceapproach and looking at all possible combinations (consisting of L^(N)combinations), an iterative approach to find the K best combinationswill now be described:

1. Sort elements in the N vectors T₁, . . . , T_(N) in descending order.Hence, after sorting, we know that the first index vector I⁽¹⁾ of thebest combination is given by I⁽¹⁾=[1, . . . , 1]. The correspondingmaximum sum is S⁽¹⁾=Σ_(n=1) ^(N)t_(n) ⁽¹⁾.

2. Compute difference vectors D_(n) for each vector T_(n), where theelements in D_(n) give the difference between two neighboring elementsT_(n), i.e. D_(n)=[t_(n) ⁽²⁾−t_(n) ⁽¹⁾, t_(n) ⁽³⁾−t_(n) ⁽²⁾, . . . ,t_(n) ^((l+1))−t_(n) ^((l)), . . . , t_(n) ^((L))−t_(n) ^((L-1))].

Note that these differences are negative due to the sorting of T_(n) indescending order.

3. Initialize “candidate sets” of vectors/scalars, which store neighborindices/neighbor sums, resp., as empty sets: Ĩ=[ ], {tilde over (S)}=[].

4. Iteratively perform for k=2, . . . , K the following two steps:

-   -   4.1 Add all possible neighbors of I^((k-1)) (vectors indices of        previously best sum in step k−1) to Ĩ and their corresponding        sums to {tilde over (S)}, respectively, if they are not already        included. A neighbor of I^((k-1)) is defined as a combination        that differs to I^((k-1)) in only one index position and, hence,        there are N neighbors for each index vector. For example, for        N=4, the N neighbors of I⁽¹⁾, which was [1,1,1,1], are given by        [2,1,1,1], [1,2,1,1], [1,1,2,1], [1,1,1,2]. The corresponding        element sums are given by adding the SNR difference from D_(n)        to S^((k-1)), without the need to compute the complete sum        again. This corresponds to subtracting the last contribution,        and adding a new (smaller) contribution instead. Note that the        elements of the difference vectors D_(n) can preferably be        computed on-the-fly, since not all entries may be needed.

In an embodiment, not all possible neighbors of I^((k-1)) (vectorsindices of previously best sum in step k−1) are added to Ĩ and theircorresponding sums to {tilde over (S)} (unless they already exist inthese sets), but pruning is applied with the following test:

Define the n-th direct neighbor of I^((k-1)) as I^((k-1))+e_(n), withe_(n) being the n-th unit vector, 1≤n≤N.

Do not add the n-th neighbor I^((k-1))+e_(n), to Ĩ, if there alreadyexists at least one candidate element in Ĩ, called I_(c), which fulfillsfor all N dimensions the following inequality: I_(c)≤I^((k-1))+e_(n).The inequality has to hold for each of the N dimensions.

4.2 Find best combination from {Ĩ,{tilde over (S)}}, i.e., the neighborcombination with the largest sum. This is the kth best combination andis removed from {Ĩ, {tilde over (S)}}.

As an example, the first steps for the case of N=4 are stated below:

1. init: Ĩ=[ ], {tilde over (S)}=[ ].

2. k=1: best sum: I⁽¹⁾=[1, . . . , 1]. Assume that S⁽¹⁾=Σ_(n=1)^(N)t_(n) ⁽¹⁾=60.

3. k=2: neighbors of I⁽¹⁾: [2,1,1,1], [1,2,1,1], [1,1,2,1], [1,1,1,2].Corresponding sums are assumed to be 39, 44, 45, 50, i.e.,t ₁ ⁽²⁾ +t ₂ ⁽¹⁾ +t ₃ ⁽¹⁾ +t ₄₀ ⁽¹⁾=39, i.e., D ₁=[t ₁ ⁽²⁾ −t ₁⁽¹⁾=60−39=21, . . . ]t ₁ ⁽¹⁾ +t ₂ ⁽²⁾ +t ₃ ⁽¹⁾ +t ₄₀ ⁽¹⁾=44, i.e., D ₂=[t ₂ ⁽²⁾ −t ₂⁽¹⁾=60−44=16, . . . ]t ₁ ⁽¹⁾ +t ₂ ⁽¹⁾ +t ₃ ⁽²⁾ +t ₄₀ ⁽¹⁾=45, i.e., D ₃=[t ₃ ⁽²⁾ −t ₃⁽¹⁾=60−45=15, . . . ]t ₁ ⁽¹⁾ +t ₂ ⁽¹⁾ +t ₃ ⁽¹⁾ +t ₄₀ ⁽²⁾=50, i.e., D ₄=[t ₄ ⁽²⁾ −t ₄⁽¹⁾=60−50=10, . . . ]Note that instead if computing the sums, only the differences can beconsidered, e.g. for the last row,t ₁ ⁽¹⁾ +t ₂ ⁽¹⁾ +t ₃ ⁽¹⁾ +t ₄₀ ⁽²⁾ =S ⁽¹⁾ +t ₄ ⁽²⁾ −t ₄ ⁽¹⁾,i.e., subtract from the last best sum the influence of the previousentry of the 4^(th) row t₄ ⁽¹⁾, and add the new (smaller) contributiont₄ ⁽²⁾ instead to it.

-   -   a. Form the “candidate sets” sets by adding these neighbors to Ĩ        and {tilde over (S)}: Ĩ=[[2,1,1,1], [1,2,1,1], [1,1,2,1],        [1,1,1,2]],    -   {tilde over (S)}=[39, 44, 45, 50].    -   b. Select best combination from “candidate sets” as second best        combination (k=2):    -   I⁽²⁾=[1,1,1,2] and S⁽²⁾=50, and subtract them from these sets        Ĩ=[[2,1,1,1], [1,2,1,1], [1,1,2,1]],    -   {tilde over (S)}=[39, 44, 45].    -   4. k=3: neighbors of I⁽²⁾: [2,1,1,2], [1,2,1,2], [1,1,2,2],        [1,1,1,3].

In an embodiment, not all neighbors will be added to Ĩ and {tilde over(S)} according to the following pruning test:

[2,1,1,2] will always have a weaker overall score than the existingcandidate [2,1,1,1], because for all N=4 dimensions, these inequalitieshave been fulfilled:

2≤2; 1≤1; 1≤1; 1≤2. The last inequality is strictly fulfilled, showingthat this candidate uses a better score on the n=4-th dimension, thusresulting in an overall better score. The candidate score is 39, whilethe potential neighbor will result in a score of 29.In the same manner, also [1,2,1,2] and [1,1,2,2] will not be added asnew neighbors, since better candidates already exist in the set Ĩ.The following description does not consider pruning:

Corresponding sums are:t ₁ ⁽²⁾ +t ₂ ⁽¹⁾ +t ₃ ⁽¹⁾ +t ₄₀ ⁽²⁾=29t ₁ ⁽¹⁾ +t ₂ ⁽²⁾ +t ₃ ⁽¹⁾ +t ₄₀ ⁽²⁾=34t ₁ ⁽¹⁾ +t ₂ ⁽¹⁾ +t ₃ ⁽²⁾ +t ₄₀ ⁽²⁾=35t ₁ ⁽¹⁾ +t ₂ ⁽¹⁾ +t ₃ ⁽¹⁾ +t ₄₀ ⁽³⁾=28, i.e., D ₄=[t ₄ ⁽²⁾ −t ₄ ⁽¹⁾=10,t ₄ ⁽³⁾ −t ₄ ⁽²⁾=22 . . . ].Again, instead of explicitly computing these sums, the elements of D_(n)may be used, e.g., for the 2^(nd) row:t ₁ ⁽¹⁾ +t ₂ ⁽¹⁾ +t ₃ ⁽¹⁾ +t ₄₀ ⁽²⁾ =S ⁽²⁾ +t ₂ ⁽²⁾ −t ₂ ⁽¹⁾,i.e., subtract from the last best sum the influence of the previousentry of the 2^(th) row t₂ ⁽¹⁾, and add the new (smaller) contributiont₂ ⁽²⁾ instead to it.

-   -   a. Form the “candidate sets” sets by adding these neighbors to Ĩ        and {tilde over (S)}: Ĩ=[[2,1,1,1], [1,2,1,1], [1,1,2,1],        [2,1,1,2], [1,2,1,2], [1,1,2,2], [1,1,1,3] ],    -   {tilde over (S)}=[39, 44, 45, 29, 34, 35, 28].    -   5. Select best combination from “candidate sets” as third best        combination (k=−3):    -   I⁽³⁾=[1,1,2,1] and S⁽³⁾=45 and subtract them from these sets        Ĩ=[[2,1,1,1], [1,2,1,1], [2,1,1,2], [1,2,1,2], [1,1,2,2],        [1,1,1,3]],    -   {tilde over (S)}=[39, 44, 29, 34, 35, 28].

In another embodiment the processing unit 32 is configured to determinean antenna score per initiator antenna array 11, 12 and per responderantenna array 21, 22 based on the first initiator quality informationand the first responder quality information of the antenna beamcombinations used in the first training stage. This is illustrated inFIG. 8. FIG. 8A shows an antenna score table 511, 512, 521, 522 perantenna array 11, 12, 21, 22. Said antenna scores, i.e. each score table511, 512, 521, 522, include a score value per antenna beam used in thefirst training stage, i.e. the scores for the different antenna beamsused by the respective antenna for transmitting a training signal. Thedetermined antenna scores may then be used for selecting the sub-set ofantenna beam combinations for use in the second training stage.

For this purpose an overall score for the different complete antennabeam combinations of antenna beams from the different initiator antennaarrays and the different responder antenna arrays. This is illustratedin FIG. 8B showing the overall score as calculated from the score tables511, 512, 521, 522 shown in FIG. 8A. The overall score may hereby becalculated as a product, sum, average or linear combination of theantenna scores. The calculated overall scores are then used forselecting the sub-set of antenna beam combinations for use in the secondtraining stage, e.g. by selecting the antenna beam combinationsproviding the highest overall scores.

In another embodiment the processing unit 32 calculates an overall scorefor different complete antenna beam combinations of antenna beams fromthe different initiator antenna arrays and the different responderantenna arrays based on the first initiator quality information and thefirst responder quality information (e.g. the scores as shown in FIG.8A) of the antenna beam combinations used in the first training stageand uses the calculated overall scores for selecting the sub-set ofantenna beam combinations for use in the second training stage.

In still another embodiment the processing unit 32 sorts the antennascores (as shown in FIG. 8A) per initiator antenna array and perresponder antenna array and selects the sub-set of antenna beamcombinations for use in the second training stage based on the sortedantenna scores by selecting the antenna beam combinations having thebest scores and/or by use of a probability distribution determined fromthe antenna scores. Further, the processing unit 32 selects antenna beamcombinations for use in the second training stage, in which one or moreantenna beams are replaced by one or more of its nearest neighbors inthe sorted antenna scores.

In another embodiment the control unit 31 is configured to repeat thefirst and/or second training stage if the signal level of thecommunication decreases or the quality of the communication decreases ora trigger is issued or time-out is reached, wherein one or more finalantenna beam combinations used earlier are used as a start for selectingantenna beam combinations in the first and/or second training phase.

In another embodiment the control unit 31 stops the first and/or secondtraining stage if a time-out is reached or a predetermined number ofantenna combinations have been tested or the improvement with respect tothe past iterations and/or with respect to the best obtained metric(such as a second quality information, e.g., MIMO capacity) decreasesbelow a predetermined threshold or the obtained metric exceeds anabsolute upper threshold.

FIG. 9 shows another embodiment of a beamforming device 30 c accordingto the present disclosure. In addition to the control unit 31 and theprocessing unit 32 it comprises a data interface 33 for—depending onwhether the beamforming device is part of the initiator 10 or theresponder 20—receiving the first responder quality information and/orthe second responder quality information from the responder 20 and/orfor transmitting the first initiator quality information and/or thesecond initiator quality information to the responder 20 and/or fortransmitting the first responder quality information and/or the secondresponder quality information to the initiator 10 and/or for receivingthe first initiator quality information and/or the second initiatorquality information from the initiator 10.

Said data interface 33 may be configured to transmit responder selectioninformation indicating the responder antenna beams of the selectedsub-set of antenna beam combinations to the responder 20 for use in thesecond training stage and/or to transmit initiator selection informationindicating the initiator antenna beams of the selected sub-set ofantenna beam combinations to the initiator 10 for use in the secondtraining stage.

In still another embodiment, in the first training stage a set ofantenna beams is tested. Then, an interpolation is performed tocalculate more antenna beam scores than actually measured, or a smalloffset is simply added for the next training stage. Subsequently,antenna beam combinations that were not covered by the beam scoremeasurement in the first training stage may be selected for the secondtraining stage, and/or a beam pointing in a slightly different directionnext to the optimum may be selected. Hence, in this embodiment, theprocessing unit selects a sub-set of the antenna beam combinations thatare derived from the antenna beam combination used in the first trainingstage for use in a second training stage.

In the following Sector Sweep (SSW) feedback (FBK) and acknowledgement(ACK) frames for use in the context of the present disclosure will bedescribed. Several possibilities are shown to include more sectorfeedback information in the SSW feedback and acknowledgement frameswithout modifying the current lengths of these (as defined inIEEE80.11ad).

One feedback frame, sent from the initiator to the responder comprisesbut is not limited to an abstract addressing field, a plurality ofsectors, each defined by a sector ID or an index determining theposition in the sequence of tested beam directions, an antenna or RFchain identifier and a score value, indicative of the first channelquality information e.g., SNR, SINR or RSSI received by the initiator ina second phase of the first training stage. Further fields may bepresent, as shown in FIGS. 11A, 13B and 13C. The number of sectors, forwhich feedback is provided, can be either fix and established in astandard or dynamic in which case a value indicative of this must becontained in the feedback frame. The sectors may be included in thedecreasing order of the score values. In a proposed feedback frame, thescore values are defined in absolute manner, as suggested in FIG. 12A Inan alternative feedback frame proposal, the scores are defined indifferential manner, whereby the difference between the score values ofthe sectors signaled in the Selected Sector Field is encoded. Accordingto another aspect, one acknowledgement frame, sent from the responder tothe initiator, comprises an abstract addressing field, a plurality ofsectors, each defined by a sector ID or an index determining theposition in the sequence of tested beam directions, an antenna or RFchain identifier and a score value indicative of the first channelquality information e.g., SNR, SINR or RSSI received by the responder ina first phase of the first training stage.

According to another embodiment it is suggested to replace the longaddressing fields currently defined in IEEE80.11ad SSW FBK and ACKframes, with an efficient compressed addressing scheme, and re-use thebits resulting thereof to include indexes and score values correspondingto multiple sectors which were received with best first channel qualityinformation in the first training stage. In this manner, the desiredinformation can be sent without increasing the length of these frames,as defined in IEEE802.11ad.

A possible construction of the SSW feedback frame starts from the legacySSW FBK Frame shown in FIG. 10A. An option is to include multiple SSWfeedback fields as depicted in FIG. 10B in the space made free byreplacing the long addressing from IEEE802.11ad with the shortaddressing used by short SSW frames. In an alternative option, only themost relevant contents of the SSW FBK Field are maintained fromIEEE802.11ad together with their bits requirements for each fed backsector and an example of the modified SSW feedback field is depicted inFIG. 11B. In legacy IEEE802.11ad, 16 bits are provided for one sector(for sector ID, DMG antenna and SNR report) for the SSW feedback fieldas shown in FIG. 10B. The possible modification is shown in FIG. 11,wherein FIG. 11A shows a modified SSW frame and FIG. 11B shows amodified SSW feedback field. With the reduction in addressing, anenlarged SSW FBK Field can be constructed, in which up to 6 sectorscould be managed to be signaled. To indicate for which receive antennathe sector and corresponding quality information signaled in the currentSSW feedback frame applies, an Antenna ID field may be defined. This canbe done e.g., by reusing reserved bits from the Reserved field in FIG.11B.

The SNR report field may contain the score values (e.g., SNR, SINR orRSSI) obtained after the first training stage. These values can beeither absolute (as shown in FIG. 12A) or differential (as shown in FIG.12B). In the absolute case i₁, . . . , i_(n) are the indexes of thesectors received with the best channel quality (i.e., the sectors havingthe largest score values) and s₁, . . . , s_(n) represent the scorevalue of i₁ and i_(n) respectively. For ease of exposition, it can beassumed that the sectors are arranged in the frame in the decreasingorder of their score values thus s₁>=s₂. In the differential case s₁, .. . , s_(Δn) represent the score value of i₁, the difference score values_(Δ2)=s₂−s₁ and s_(Δn)=s_(n)−s_(n-1). For the differential score case,the number of bits required may be reduced as the difference between twoscores is expected to be significantly smaller than the total rangedefined in the standard. The length required for the differential scorescan be either fixed or signaled. For the second option, the valuesignaled can be indicative of the number of bits required for thelargest relevant score difference. Then all differential scores can bedefined to use this length.

A further possible SSW FBK frame may consider variable number of sectorswhich are fed back. Such a scheme may be useful when one antenna array(e.g., 11 as depicted in FIG. 5) receives a strong training signalwhereas a different one (e.g., 12) receives a significantly attenuatedone. According to one aspect presented herein, the joint score based onwhich the beams in the second training stage are chosen, givespreference to the strong sectors. Thus, one antenna array will have moresectors relevant for the joint metric than the other one. To accommodatethis, a possible SSW FBK, as depicted in FIG. 15A may additionallycontain a field indicating the number of sectors, for which beam indexinformation as well as score values are included.

Another option suggests, for SSW feedback frame and SSW ACK in DTI, tokeep the SSW feedback the same length but use the short SSW framestructure as baseline. Similar to the structure shown in FIG. 11, afirst possible approach to include information about multiple sectors isto reduce the addressing to the short addressing, used by the short SSWframes and, in the space thus emptied, pack several single sectorfeedback fields. Each single SSW feedback field contains in this caseCDOWN fields. SNR fields and RF chain ID, polling bit and reserved bitswhich apply to one received sector. The reserved bits can be redefinedto provide information about polarization or bandwidth. Alternative tohaving multiple single SSW feedback fields as previously described, astructure as depicted in FIG. 13A can be used, where only the sectorindexes, SNR values and possibly RF Chain IDs are sent for each sector.If the SNR is still 8 bits, one sector requires 19 bits to signal, inthe CDOWN field the number of the sector in the sweep phase and in theRF Chain ID field, the corresponding RF chain ID from which the sectorwas sent. An option for the modified SSW feedback is suggested in FIG.13B where the CDOWN and RF Chain ID fields are extended to n sectors.The difference between the options shown in FIGS. 13A and 13C are onlyin the control fields following after BRP Request i.e., FIG. 13A usesthe control fields which are defined in IEEE802.11ad SSW FBK Frame afterBRP Request, whereas FIG. 13C uses control fields defined in the shortSSW frame. However, based on the option shown in FIG. 13A up to 7sectors may be signaled whereas with option shown in FIG. 13C up to 10sectors may be signaled. To indicate for which receive antenna thesector and corresponding quality information signaled in the current SSWfeedback frame applies, an RX RF Chain ID field may be defined. Thisfield can be present as separate field as shown in FIGS. 13A-C or byreusing bits of the Reserved field. The number of bits left reserveddepends on the number of sectors that have been signaled, therefore itis marked in FIGS. 13A-13C as a function “L” of “n”.

With the options shown in FIGS. 13A to 13C, information about “n”sectors could be signaled. To keep the same number of bits for theentire frame, reserved bits may be defined or the BRP Request fields maythe extended. For example if “n=7” sectors can be fed back, one bit canbe either defined as reserved or it can be appended to an extended BRPRequest. Due to increased supported features in IEEE 802.11ay, e.g.,support for polarization or channel bonding, the content of the BRPRequest may be subject to change, thus the latter option of using theremaining bits for redefining this field may be beneficial. Similarly,if e.g., “n=6” sectors are fed back, then the number of remaining bitsis 8 and again they can be either defined as reserved or extended BRP.Clearly, values shown here are only meant to give an indication of thenumber of sectors which may be signaled with this proposal.

The same options as discussed with respect to the relative and absoluteor differential score values above may also apply to the options for SSWFBK and SSW ACK in DTI as discussed with respect to FIGS. 13A to 13Cwith the difference that sector information is in the CDOWN field. Thisis illustrated in FIG. 14A (showing an SSW feedback field for theabsolute case) and FIG. 14B (showing an SSW feedback field for thedifferential case).

The same options as discussed with respect to signaling a variablenumber of sectors may also apply. This option is depicted in FIGS. 15Band 15C, as a possible SSW FBK, which additionally contains a fieldindicating the number of sectors, for which beam index information aswell as score values are included. The number of bits required for thiscan be either obtained due to reductions in differential SNR, reusingreserved bits or various tradeoffs between length of the refinementrequest fields and the multiple sector related fields.

The three options considered for the SSW FBK frame apply also for theSSW ACK frame, with the principal difference that in the case of thelatter multiple sectors which have been received with best first qualityinformation by the responder, together with the corresponding scorevalues are signaled to the initiator.

Having the same frame length as described in 802.11ad can be beneficialfor ensuring interoperability with legacy devices, however it may not beenough for the requirements. e.g. of a new standard. Therefore severalalternatives to the frame structures presented so far in this disclosureare also shown.

Should the number of sectors received at a device with a channel quality(e.g., SNR/SINR/RSSI) larger than a threshold, exceed the number ofsectors which can be included within an SSW FBK frame or should thedevice be asked to feedback a larger number of sectors that can beincluded within an SSW FBK or SSW ACK frame, then several solutions canbe envisaged. One is to transmit the remaining feedback fieldinformation in a separate frame, which is however part of the samecontrol PHY PPDU. An example of such a frame is a control trailer.

More clearly, for multiple sectors the feedback frame depicted in FIG.11A is kept the same. However, in an additional frame (e.g. a controltrailer) a succession of the fields Selected Sectors, DMG Antenna andSNR Report as shown in FIG. 11B, will contain the information on theadditional sectors, which could not be accommodated in the SSW feedbackframe, together with an indication of their number. Similarly, if theSSW feedback frame is the one used in FIG. 13A the information in thecontrol trailer will contain a succession of the fields CDOWN and SINRReports as shown in FIG. 14A or 14B and an indication of the number ofsectors present. The SNR reports in the control trailer may be eitherabsolute as shown in FIG. 14A or differential as shown in FIG. 14B.

A more robust type of SSW FBK/SSW ACK structure to allow for largenumber of multiple sectors being fed back can be defined as shown inFIG. 16, where a fragmented bit (field “Fragmented bit”) indicates thatthere are more sectors being signaled in this separate frame e.g., thecontrol trailer. The significance of the other fields present in FIG. 16is the same as the ones in the SSW frame shown in FIG. 13A.

According to an alternative to the SSW FBK/SSW ACK frame presented inFIG. 16 the SSW frame includes not only the fragmented bit, but also anindication (field “N_sectors”) of the number of sectors that will bepresent in the additional frame e.g., the control trailer or of thetotal number of sectors that have been received with a good channelquality. An exemplary SSW frame according to this embodiment is shown inFIG. 17.

The solutions using a Fragmented bit to indicate the continuation of thefeedback information in a separate frame within the same PPDU has beenpresented above and in FIGS. 16 and 17 for the proposed SSW FBK/SSW ACKshown in FIG. 13B, however it can be similarly applied to otherstructures, e.g. the structures shown in FIG. 11.

Alternative to feeding back the remaining sector information in acontrol trailer, is signaling these sectors in a structure alreadydefined in the current 802.11 ad standard as TxSS Sector List. In thiscase the fragmented bit and/or N_sectors defined in FIGS. 16, 17 can bedefined to signal that the multiple sectors or the respective number ofsectors is fed back within this structure.

An alternative proposal implies having a reduced SSW FBK/SSW ACK frame,in which the short addressing is again used, however only informationabout one sector is included, whereas all the information about themultiple sectors is signaled in the additional structure. e.g., thecontrol trailer. This can be seen as a special case of the framepresented in FIG. 16, where n=1 holds for the SSW Feedback Field shownin FIG. 13B. In this way space within the data field of a control PPDUcan be spared for other control information. The Fragmented bit and theindication of number of sectors can be either in separate fields asshown in FIGS. 16, 17 or they can be placed instead of what currently isdefined as reserved bits.

An alternative is defining as a reply to a sector sweep SSW FBK/SSW ACKas structures with variable length, which contain among other elements anumber of measurements for the sectors that are above a certainthreshold, sector identification, antenna identification and valueindicating channel quality information e.g., SNR/SINR/RSSI etc.

It should be noted that the ways of providing feedback from theresponder to the initiator and/or from the initiator to the responder,in particular the use of SSW feedback frames and/or feedback fields asillustrated in FIGS. 11 to 17, are not restricted to be used in thecontext of a beamforming device, beamforming method, initiator,responder, communication device and communication system as disclosedherein in the context of FIGS. 1 to 9, but can also be used with otherbeamforming devices, beamforming methods, initiators, responders,communication devices and communication systems. Generally, in anydevices and methods using beamforming for use in a wirelesscommunication system, particularly comprising an initiator and aresponder, in which feedback is provided from one communication deviceto another communication device, such SSW feedback frames and/orfeedback fields as disclosed herein may be applied.

The disclosed approaches are well suited to be adopted by further802.11ay products because they fit the proposed architecture for thefuture 802.11ay standard in which arrays of sub-arrays are employed andthey require only slight modifications to the 802.11ad frame structuresand beamforming/training procedures, which is a desired feature for theupcoming 802.11ay standard.

Compared to the known pairwise search algorithm, the disclosedapproaches benefit from a significant reduction in complexity whichleads to a decrease in time needed to initiate a high rate data linkbetween initiator and responder. The performance of the disclosedapproaches is similar as that of pairwise search and both have anegligible loss compared with brute-force full exhaustive search (about1% or less capacity loss).

Each time, beams are tested (trained), a training sequence istransmitted, and channel estimation may be performed at the receivingend, which is time consuming. Depending on the geometry and otherproperties of the antenna arrays, the beamwidths may become very small,e.g. 30° directed beams. This in turn results in a large amount ofpotential sectors, into which the conventional SLS procedure may sweep,e.g. 49 sectors for each antenna array. In a 2×2 case, exhaustivetraining would then result in 49⁴=approximately 5.76 million trainingprocedures. This number is drastically reduced by the disclosedapproaches.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present disclosure. As will be understood by thoseskilled in the art, the present disclosure may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosure of the presentdisclosure is intended to be illustrative, but not limiting of the scopeof the disclosure, as well as other claims. The disclosure, includingany readily discernible variants of the teachings herein, defines, inpart, the scope of the foregoing claim terminology such that noinventive subject matter is dedicated to the public.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. A single element or other unit may fulfill the functions ofseveral items recited in the claims. The mere fact that certain measuresare recited in mutually different dependent claims does not indicatethat a combination of these measures cannot be used to advantage.

In so far as embodiments of the disclosure have been described as beingimplemented, at least in part, by software-controlled data processingapparatus, it will be appreciated that a non-transitory machine-readablemedium carrying such software, such as an optical disk, a magnetic disk,semiconductor memory or the like, is also considered to represent anembodiment of the present disclosure. Further, such a software may alsobe distributed in other forms, such as via the Internet or other wiredor wireless telecommunication systems.

The elements of the disclosed devices, apparatus and systems may beimplemented by corresponding hardware and/or software elements, forinstance appropriated circuits. A circuit is a structural assemblage ofelectronic components including conventional circuit elements,integrated circuits including application specific integrated circuits,standard integrated circuits, application specific standard products,and field programmable gate arrays. Further a circuit includes centralprocessing units, graphics processing units, and microprocessors whichare programmed or configured according to software code. A circuit doesnot include pure software, although a circuit includes theabove-described hardware executing software.

It follows a list of further embodiments of the disclosed subjectmatter:

1. A beamforming device for use in a wireless communication systemcomprising an initiator and a responder, said beamforming devicecomprising:

-   -   a control unit (31) for controlling the initiator (10) having        one or more initiator antenna arrays (11, 12) each comprising        two or more initiator antenna elements (110, 120) and/or the        responder (20) having one or more responder antenna arrays (21,        22) each comprising two or more responder antenna elements (210,        220), wherein the initiator (10) has at least two initiator        antenna arrays (11, 12) and/or the responder (20) has at least        two responder antenna arrays (21, 22),    -   a processing unit (32) for selecting one or more initiator        antenna beams per initiator antenna array (11, 12) and one or        more responder antenna beams per responder antenna array (21,        22) for use by the initiator (10) and the responder (20) in        communicating with each other,

wherein said control unit (31) and said processing unit (32) areconfigured

i) to control, by the control unit (31) in a first training stage, theinitiator antenna elements (110, 120), per pair of initiator antennaarray (11, 12) and responder antenna array (21, 22), to transmit a firsttraining signal by successively using different initiator antenna beamsof different initiator antenna beam directions and to receive a firsttraining signal transmitted by the responder antenna array (21, 22) bysuccessively using different responder antenna beams of differentresponder antenna beam directions,ii) to select, by the processing unit (32), a sub-set of the antennabeam combinations that have been used in the first training stage or arederived from the antenna beam combination used in the first trainingstage, for use in a second training stage, wherein at least some of theantenna beam combinations of said sub-set are selected by use of firstresponder quality information indicating the quality of reception of thefirst training signals by the respective responder antenna array for thedifferent initiator antenna beams used by the respective initiatorantenna array for transmitting the first training signal and firstinitiator quality information indicating the quality of reception of thefirst training signals by the respective initiator antenna array for thedifferent responder antenna beams used by the respective responderantenna array for transmitting the first training signal in the firsttraining stage, wherein the first responder quality information and thefirst initiator quality information obtained for the different pairs ofinitiator antenna arrays and responder antenna arrays of the firsttraining stage is used,iii) to control, by the control unit (31) in the second training stage,the initiator antenna elements (110, 120) of the initiator antennaarrays (11, 12) to transmit, in particular commonly (simultaneously orsuccessively), a second training signal by successively using differentinitiator antenna beams of different initiator antenna beam directionsaccording to one or more of the selected antenna beam combinations,and/or to receive a second training signal transmitted, in particularcommonly (simultaneously or successively), by the responder antennaelements (210, 220) of the one or more responder antenna arrays (21, 22)by successively using different responder antenna beams of differentresponder antenna beam directions according to one or more of theselected antenna beam combinations, andiv) to select, by the processing unit (32), a final antenna beamcombination for use by the initiator (10) and the responder (20) incommunicating with each other from second responder quality informationindicating the quality of reception of the second training signals bythe responder antenna arrays for the different initiator antenna beamsused by the initiator antenna arrays for transmitting the secondtraining signal and/or from second initiator quality informationindicating the quality of reception of the second training signals bythe initiator antenna arrays for the different responder antenna beamsused by the responder antenna arrays for transmitting the secondtraining signal in the second training stage.2. The beamforming device as defined in embodiment 1,wherein said control unit (31) is configured to control, in the firsttraining stage, the initiator antenna elements (110, 120), per pair ofinitiator antenna array (11, 12) and responder antenna array (21, 22),

-   -   to transmit the first training signal by successively using        different initiator antenna beams of different beam directions        to a responder antenna array configured to receive the first        training signal with an omnidirectional or wide-angle responder        antenna beam and    -   to receive a first training signal transmitted by a responder        antenna array (21, 22) by successively using different responder        antenna beams of different beam directions, wherein the        initiator antenna array (11, 12) is configured to receive the        first training signal with an omnidirectional or wide-angle        initiator antenna beam.        3. The beamforming device as defined in any preceding        embodiment,        wherein said control unit (31) is configured to control, in the        first training stage, the initiator antenna elements (110, 120),        per pair of initiator antenna array (11, 12) and responder        antenna array (21, 22),    -   to transmit the first training signal by successively using        different initiator antenna beams of different initiator antenna        beam directions to a responder antenna configured to receive the        first training signal with a directed responder antenna beam,        wherein the successive transmission of the first training signal        with different initiator antenna beams of different initiator        antenna beam directions is repeated multiple times, wherein in        each iteration one or more different directed responder antenna        beams are used for reception, and    -   to receive the first training signal transmitted from a        responder antenna array (21, 22) by successively using different        responder antenna beams of different responder antenna beam        directions, wherein the initiator antenna array (11, 12) is        configured to receive the first training signal with a directed        initiator antenna beam, wherein the successive transmission of        the first training signal with different responder antenna beams        of different responder antenna beam directions is repeated        multiple times, wherein in each iteration one or more different        directed initiator antenna beams are used for reception.        4. The beamforming device as defined in any preceding        embodiment,        wherein said control unit (31) is configured to control, in a        first training stage, the initiator antenna elements (110, 120),        per two or more pairs of initiator antenna arrays (11, 12) and        responder antenna arrays (21, 22),    -   to transmit a first training signal successively with different        initiator antenna beams of different initiator antenna beam        directions to a responder antenna, wherein the initiator antenna        elements of different initiator antenna arrays simultaneously        transmit orthogonal first training signals and/or over different        polarization, and    -   to receive a first training signal transmitted from a responder        antenna array (21, 22) successively with different responder        antenna beams of different responder antenna beam directions,        wherein the responder antenna elements of different responder        antenna arrays simultaneously transmit orthogonal first training        signals and/or over different polarization.        5. The beamforming device as defined in any preceding        embodiment,        wherein said control unit (31) and said processing unit (32) are        configured    -   to control, by the control unit (31), in a first phase of the        first and/or second training stage, the initiator antenna        elements (110, 120) to transmit the training signal by        successively using different initiator antenna beams having a        first beam width and/or the responder antenna elements (210,        220) to receive the training signal by successively using        different responder antenna beams having a first beam width,    -   to select, by the processing unit (32), at least part of the        antenna beam combinations of the sub-set of antenna beam        combinations for use in a second phase of the same training        stage based on the initiator antenna beam direction of the        initiator antenna beam selected based on the responder quality        information, and    -   to control, by the control unit (31), in the second phase, the        initiator antenna elements (110, 120) to transmit the training        signal by successively using different initiator antenna beams        having a second beam width different from the first beam width        used in the first phase and having an initiator antenna beam        direction identical or similar as the initiator antenna beam        direction of the initiator antenna beam providing the best first        initiator quality information in the first phase and/or the        responder antenna elements (210, 220) to receive the training        signal by successively using different responder antenna beams        having a second beam width different from the first beam width        used in the first phase and having a responder antenna beam        direction identical or similar as the responder antenna beam        direction of the responder antenna beam providing the best first        responder quality information in the first phase.        6. A beamforming device for use in a wireless communication        system comprising an initiator and a responder, said beamforming        device comprising:    -   a control unit (31) for controlling the initiator (10) having        one or more initiator antenna arrays (11, 12) each comprising        two or more initiator antenna elements (110, 120) and/or the        responder (20) having one or more responder antenna arrays (21,        22) each comprising two or more responder antenna elements (210,        220), wherein the initiator (10) has at least two initiator        antenna arrays (11, 12) and/or the responder (20) has at least        two responder antenna arrays (21, 22),    -   a processing unit (32) for selecting one or more initiator        antenna beams per initiator antenna array (11, 12) and one or        more responder antenna beams per responder antenna array (21,        22) for use by the initiator (10) and the responder (20) in        communicating with each other,    -   wherein said control unit (31) and said processing unit (32) are        configured        i) to control, by the control unit (31) in a first training        stage, the responder antenna elements (210, 220), per pair of        initiator antenna array (11, 12) and responder antenna array        (21, 22) to receive a first training signal transmitted by the        initiator antenna array (11, 12) by successively using different        initiator antenna beams of different initiator antenna beam        directions and to transmit a first training signal by        successively using different responder antenna beams of        different responder antenna beam directions,        ii) to select, by the processing unit (32), a sub-set of the        antenna beam combinations that have been used in the first        training stage or are derived from the antenna beam combination        used in the first training stage, for use in a second training        stage, wherein at least some of the antenna beam combinations of        said sub-set are selected by use of first responder quality        information indicating the quality of reception of the first        training signals by the respective responder antenna array for        the different initiator antenna beams used by the respective        initiator antenna array for transmitting the first training        signal and first initiator quality information indicating the        quality of reception of the first training signals by the        respective initiator antenna array for the different responder        antenna beams used by the respective responder antenna array for        transmitting the first training signal in the first training        stage, wherein the first responder quality information and the        first initiator quality information obtained for the different        pairs of initiator antenna arrays and responder antenna arrays        of the first training stage is used,        iii) to control, by the control unit (31) in the second training        stage, the responder antenna elements (210, 220) of the        responder antenna arrays (21, 22) to receive a second training        signal transmitted, in particular commonly (simultaneously or        successively), by the initiator antenna elements (110, 120) of        the one or more initiator antenna arrays (11, 12) by        successively using different initiator antenna beams of        different initiator antenna beam directions according to one or        more of the selected antenna beam combinations and/or to        transmit, in particular commonly (simultaneously or        successively), a second training signal by successively using        different initiator antenna beams of different initiator antenna        beam directions according to one or more of the selected antenna        beam combinations, and        iv) to select, by the processing unit (32), a final antenna beam        combination for use by the initiator (10) and the responder (20)        in communicating with each other from second responder quality        information indicating the quality of reception of the second        training signals by the responder antenna arrays for the        different initiator antenna beams used by the initiator antenna        arrays for transmitting the second training signal and/or from        second initiator quality information indicating the quality of        reception of the second training signals by the initiator        antenna arrays for the different responder antenna beams used by        the responder antenna arrays for transmitting the second        training signal in the second training stage.        7. The beamforming device as defined in any preceding        embodiment,        wherein said processing unit (32) is configured to select, per        pair of initiator antenna array (11, 12) and responder antenna        array (21, 22), combinations of initiator antenna beams and        responder antenna beams to be used in the first training stage        by use of a genetic or evolutionary search algorithm.        8. The beamforming device as defined in any preceding        embodiment,        wherein the first initiator quality information and the first        responder quality information is information indicating signal        to noise ratio, signal to noise-and-interference ratio, receive        signal strength indication, the estimated capacity, received        electric or magnetic field strength or delay spread per pair of        initiator antenna array (11, 12) and responder antenna array        (21, 22) and per antenna beam.        9. The beamforming device as defined in any preceding        embodiment,        wherein the second initiator quality information and/or the        second responder quality information is information indicating        the estimated capacity, sum of singular values, or condition        number of a channel matrix per antenna beam combination.        10. The beamforming device as defined in any preceding        embodiment,        wherein said processing unit (32) is configured to determine an        antenna score per initiator antenna array and per responder        antenna array (21, 22) based on the first initiator quality        information and the first responder quality information of the        antenna beam combinations used in the first training stage, said        antenna scores including a score value per antenna beam used in        the first training stage, and to use the determined antenna        scores for selecting the sub-set of antenna beam combinations        for use in the second training stage.        11. The beamforming device as defined in embodiment 10,        wherein said processing unit (32) is configured to calculate an        overall score for the different complete antenna beam        combinations of antenna beams from the different initiator        antenna arrays and the different responder antenna arrays as a        product, sum, average or linear combination of the antenna        scores and to use the calculated overall scores for selecting        the sub-set of antenna beam combinations for use in the second        training stage.        12. The beamforming device as defined in any preceding        embodiment,        wherein said processing unit (32) is configured to calculate an        overall score for different complete antenna beam combinations        of antenna beams from the different initiator antenna arrays and        the different responder antenna arrays based on the first        initiator quality information and the first responder quality        information of the antenna beam combinations used in the first        training stage and to use the calculated overall scores for        selecting the sub-set of antenna beam combinations for use in        the second training stage.        13. The beamforming device as defined in embodiment 10,        wherein said processing unit (32) is configured to sort the        antenna scores per initiator antenna array and per responder        antenna array and to select the sub-set of antenna beam        combinations for use in the second training stage based on the        sorted antenna scores by selecting the antenna beam combinations        having the best scores and/or by use of a probability        distribution determined from the antenna scores, and to select        antenna beam combinations for use in the second training stage,        in which one or more antenna beams are replaced by one or more        of its nearest neighbors in the sorted antenna scores.        14. The beamforming device as defined in any preceding        embodiment,        wherein said processing unit (32) is configured to select the        sub-set of antenna beam combinations for use in the second        training stage by use of a genetic or evolutionary search        algorithm.        15. The beamforming device as defined in embodiment 14,        wherein said processing unit (32) is configured to select part        of the antenna beam combinations of the sub-set for use in the        second training stage randomly, in particular by use of a        uniform or non-uniform probability distribution.        16. The beamforming device as defined in embodiment 14,        wherein said processing unit (32) is configured to determine, in        each iteration of step iv) in the second training stage, the        second responder quality information and/or the second initiator        quality information, to determine an overall score for the        overall antenna beam combination used in said iteration and to        compare the determined overall score with the overall score of        the previous iterations.        17. The beamforming device as defined in embodiment 16,        wherein said processing unit (32) is configured to set, in each        iteration, the overall antenna beam combination having the best        overall score up to said iteration, as preliminary best antenna        beam combination and to select the overall antenna beam        combination to be used in the next iteration based on the        preliminary best antenna beam combination.        18. The beamforming device as defined in any preceding        embodiment,        wherein said processing unit (32) is configured to use the beam        direction of the initiator antenna beam and/or the responder        antenna beam providing the best initiator quality information        and the best responder quality information for selecting antenna        beam combinations used subsequently in the first training stage        and/or the second training stage.        19. The beamforming device as defined in any preceding        embodiment,        wherein said control unit (31) is configured to repeat the first        and/or second training stage if the signal level of the        communication decreases or the quality of the communication        decreases or a trigger is issued or time-out is reached, wherein        one or more final antenna beam combinations used earlier are        used as a start for selecting antenna beam combinations in the        first and/or second training phase.        20. The beamforming device as defined in any preceding        embodiment,        wherein said control unit (31) is configured to stop the first        and/or second training stage if a time-out is reached or a        predetermined number of antenna combinations have been tested        the improvement with respect to the past iterations and/or with        respect to the best obtained metric decreases below a        predetermined threshold or the obtained metric exceeds an        absolute upper threshold.        21. The beamforming device as defined in any preceding        embodiment,        further comprising a data interface (33) for receiving the first        responder quality information and/or the second responder        quality information from the responder (20) and/or for        transmitting the first initiator quality information and/or the        second initiator quality information to the responder (20)        and/or for transmitting the first responder quality information        and/or the second responder quality information to the initiator        (10) and/or for receiving the first initiator quality        information and/or the second initiator quality information from        the initiator (10).        22. The beamforming device as defined in embodiment 21,        wherein said data interface (33) is configured to transmit        responder selection information indicating the responder antenna        beams of the selected sub-set of antenna beam combinations to        the responder (20) for use in the second training stage and/or        to transmit initiator selection information indicating the        initiator antenna beams of the selected sub-set of antenna beam        combinations to the initiator (10) for use in the second        training stage.        23. The beamforming device as defined in embodiment 21 or 22,        wherein control unit (31) is configured to control said        initiator antenna elements (110, 120) to transmit initiator        quality information and/or responder selection information        within or along with first and/or second training signals        transmitted to the responder (20) and/or to control said        responder antenna elements (210, 220) to transmit responder        quality information and/or initiator selection information        within or along with first and/or second training signals        transmitted to the initiator (10).        24. The beamforming device as defined in any preceding        embodiment,        wherein said processing unit (32) is configured to calculate one        or more of the first responder quality information, the second        responder quality information, the first initiator quality        information and the second initiator quality information.        25. A beamforming method for use in a wireless communication        system comprising an initiator and a responder, the initiator        (10) having one or more initiator antenna arrays (11, 12) each        comprising two or more initiator antenna elements (110, 120)        and/or the responder (20) having one or more responder antenna        arrays (21, 22) each comprising two or more responder antenna        elements (210, 220), wherein the initiator (10) has at least two        initiator antenna arrays (11, 12) and/or the responder (20) has        at least two responder antenna arrays (21, 22),        said beamforming method being configured for selecting one or        more initiator antenna beams per initiator antenna array (11,        12) and one or more responder antenna beams per responder        antenna array (21, 22) for use by the initiator (10) and the        responder (20) in communicating with each other,        said beamforming method comprising:        i) controlling, in a first training stage, the initiator antenna        elements (110, 120), per pair of initiator antenna array (11,        12) and responder antenna array (21, 22), to transmit a first        training signal by successively using different initiator        antenna beams of different initiator antenna beam directions and        to receive a first training signal transmitted by the responder        antenna array (21, 22) by successively using different responder        antenna beams of different responder antenna beam directions,        ii) selecting a sub-set of the antenna beam combinations that        have been used in the first training stage or are derived from        the antenna beam combination used in the first training stage,        for use in a second training stage, wherein at least some of the        antenna beam combinations of said sub-set are selected by use of        first responder quality information indicating the quality of        reception of the first training signals by the respective        responder antenna array for the different initiator antenna        beams used by the respective initiator antenna array for        transmitting the first training signal and first initiator        quality information indicating the quality of reception of the        first training signals by the respective initiator antenna array        for the different responder antenna beams used by the respective        responder antenna array for transmitting the first training        signal in the first training stage, wherein the first responder        quality information and the first initiator quality information        obtained for the different pairs of initiator antenna arrays and        responder antenna arrays of the first training stage is used,        iii) controlling, in the second training stage, the initiator        antenna elements (110, 120) of the initiator antenna arrays (11,        12) to transmit, in particular commonly (simultaneously or        successively), a second training signal by successively using        different initiator antenna beams of different initiator antenna        beam directions according to the selected antenna beam        combinations, and/or to receive a second training signal        transmitted, in particular commonly (simultaneously or        successively), by the responder antenna elements (210, 220) of        the one or more responder antenna arrays (21, 22) by        successively using different responder antenna beams of        different responder antenna beam directions according to the        selected antenna beam combinations, and        iv) selecting a final antenna beam combination for use by the        initiator (10) and the responder (20) in communicating with each        other from second responder quality information indicating the        quality of reception of the second training signals by the        responder antenna arrays for the different initiator antenna        beams used by the initiator antenna arrays for transmitting the        second training signal and/or from second initiator quality        information indicating the quality of reception of the second        training signals by the initiator antenna arrays for the        different responder antenna beams used by the responder antenna        arrays for transmitting the second training signal in the second        training stage.        26. A beamforming method for use in a wireless communication        system comprising an initiator and a responder, the initiator        (10) having one or more initiator antenna arrays (11, 12) each        comprising two or more initiator antenna elements (110, 120)        and/or the responder (20) having one or more responder antenna        arrays (21, 22) each comprising two or more responder antenna        elements (210, 220), wherein the initiator (10) has at least two        initiator antenna arrays (11, 12) and/or the responder (20) has        at least two responder antenna arrays (21, 22),        said beamforming method being configured for selecting one or        more initiator antenna beams per initiator antenna array (11,        12) and one or more responder antenna beams per responder        antenna array (21, 22) for use by the initiator (10) and the        responder (20) in communicating with each other,        said beamforming method comprising:        i) controlling, in a first training stage, the responder antenna        elements (210, 220), per pair of initiator antenna array (11,        12) and responder antenna array (21, 22) to receive a first        training signal transmitted by the initiator antenna array (11,        12) by successively using different initiator antenna beams of        different initiator antenna beam directions and to transmit a        first training signal by successively using different responder        antenna beams of different responder antenna beam directions,        ii) selecting a sub-set of the antenna beam combinations that        have been used in the first training stage or are derived from        the antenna beam combination used in the first training stage,        for use in a second training stage, wherein at least some of the        antenna beam combinations of said sub-set are selected by use of        first responder quality information indicating the quality of        reception of the first training signals by the respective        responder antenna array for the different initiator antenna        beams used by the respective initiator antenna array for        transmitting the first training signal and first initiator        quality information indicating the quality of reception of the        first training signals by the respective initiator antenna array        for the different responder antenna beams used by the respective        responder antenna array for transmitting the first training        signal in the first training stage, wherein the first responder        quality information and the first initiator quality information        obtained for the different pairs of initiator antenna arrays and        responder antenna arrays of the first training stage is used,        iii) controlling, in the second training stage, the responder        antenna elements (210, 220) of the responder antenna arrays (21,        22) to receive a second training signal transmitted, in        particular commonly (simultaneously or successively), by the        initiator antenna elements (110, 120) of the one or more        initiator antenna arrays (11, 12) by successively using        different initiator antenna beams of different initiator antenna        beam directions according to the selected antenna beam        combinations and/or to transmit, in particular commonly        (simultaneously or successively), a second training signal by        successively using different initiator antenna beams of        different initiator antenna beam directions according to the        selected antenna beam combinations, and        iv) selecting a final antenna beam combination for use by the        initiator (10) and the responder (20) in communicating with each        other from second responder quality information indicating the        quality of reception of the second training signals by the        responder antenna arrays for the different initiator antenna        beams used by the initiator antenna arrays for transmitting the        second training signal and/or from second initiator quality        information indicating the quality of reception of the second        training signals by the initiator antenna arrays for the        different responder antenna beams used by the responder antenna        arrays for transmitting the second training signal in the second        training stage.        27. A communication device (40, 41) for communicating with        another communication device in a wireless communication system,        said communication device comprising:    -   one or more antenna arrays (11, 12, 21, 22) each comprising two        or more antenna elements (110, 120, 210, 220), and    -   a beamforming device (30, 30 a, 30 b) as defined in any one of        embodiments 1 to 24.        28. A communication system including    -   a beamforming device (30, 30 a, 30 b) as defined in any one of        embodiments 1 to 24 and    -   two or more communication devices (10, 20), each having at least        one antenna array (11, 12, 21, 22) each comprising two or more        antenna elements (110, 120, 210, 220), wherein at least one        communication device has at least two antenna arrays.        29. A non-transitory computer-readable recording medium that        stores therein a computer program product, which, when executed        by a processor, causes the beamforming method according to        embodiment 25 or 26 to be performed.        30. A computer program comprising program code means for causing        a computer to perform the steps of said method according to        embodiment 25 or 26 when said computer program is carried out on        a computer.        31. The beamforming device as defined in embodiment 6,        wherein said control unit (31) is configured to control, in the        first training stage, the responder antenna elements (210, 220),        per pair of initiator antenna array (11, 12) and responder        antenna array (21, 22),    -   to transmit the first training signal by successively using        different responder antenna beams of different beam directions        to an initiator antenna array configured to receive the first        training signal with an omnidirectional or wide-angle initiator        antenna beam and    -   to receive a first training signal transmitted by an initiator        antenna array (11, 12) by successively using different initiator        antenna beams of different beam directions, wherein the        responder antenna array (21, 22) is configured to receive the        first training signal with an omnidirectional or wide-angle        responder antenna beam.        32. The beamforming device as defined in embodiment 6 or 31,        wherein said control unit (31) is configured to control, in the        first training stage, the responder antenna elements (210, 220),        per pair of initiator antenna array (11, 12) and responder        antenna array (21, 22),    -   to transmit the first training signal by successively using        different responder antenna beams of different responder antenna        beam directions to an initiator antenna configured to receive        the first training signal with a directed initiator antenna        beam, wherein the successive transmission of the first training        signal with different responder antenna beams of different        responder antenna beam directions is repeated multiple times,        wherein in each iteration one or more different directed        initiator antenna beams are used for reception, and    -   to receive the first training signal transmitted from an        initiator antenna array (11, 12) by successively using different        initiator antenna beams of different initiator antenna beam        directions, wherein the responder antenna array (21, 22) is        configured to receive the first training signal with a directed        responder antenna beam, wherein the successive transmission of        the first training signal with different initiator antenna beams        of different initiator antenna beam directions is repeated        multiple times, wherein in each iteration one or more different        directed responder antenna beams are used for reception.        33. The beamforming device as defined in embodiment 6, 31 or 32,        wherein said control unit (31) is configured to control, in a        first training stage, the responder antenna elements (210, 220),        per two or more pairs of initiator antenna arrays (11, 12) and        responder antenna arrays (21, 22),    -   to transmit a first training signal successively with different        responder antenna beams of different responder antenna beam        directions to an initiator antenna, wherein the responder        antenna elements of different responder antenna arrays        simultaneously transmit orthogonal first training signals and/or        over different polarization, and    -   to receive a first training signal transmitted from an initiator        antenna array (11, 12) successively with different initiator        antenna beams of different initiator antenna beam directions,        wherein the initiator antenna elements of different initiator        antenna arrays simultaneously transmit orthogonal first training        signals and/or over different polarization.        34. The beamforming device as defined in embodiment 6, 31, 32,        33 or 34,        wherein said control unit (31) and said processing unit (32) are        configured    -   to control, by the control unit (31), in a first phase of the        first and/or second training stage, the responder antenna        elements (210, 220) to transmit the training signal by        successively using different responder antenna beams having a        first beam width and/or the initiator antenna elements (110,        120) to receive the training signal by successively using        different initiator antenna beams having a first beam width,    -   to select, by the processing unit (32), at least part of the        antenna beam combinations of the sub-set of antenna beam        combinations for use in a second phase of the same training        stage based on the responder antenna beam direction of the        responder antenna beam selected based on the initiator quality        information, and    -   to control, by the control unit (31), in the second phase, the        responder antenna elements (210, 220) to transmit the training        signal by successively using different responder antenna beams        having a second beam width different from the first beam width        used in the first phase and having a responder antenna beam        direction identical or similar as the responder antenna beam        direction of the responder antenna beam providing the best first        responder quality information in the first phase and/or the        initiator antenna elements (110, 120) to receive the training        signal by successively using different initiator antenna beams        having a second beam width different from the first beam width        used in the first phase and having an initiator antenna beam        direction identical or similar as the initiator antenna beam        direction of the initiator antenna beam providing the best first        initiator quality information in the first phase.        35. The beamforming device as defined in embodiment 1,        wherein said control unit (31) is configured to control, in the        first training stage, the initiator antenna elements (110, 120),        per pair of initiator antenna array (11, 12) and responder        antenna array (21, 22),    -   to transmit the first training signal by successively using        different initiator antenna beams of different beam directions        to a responder antenna array configured to receive the first        training signal with an omnidirectional or wide-angle responder        antenna beam and    -   subsequently to transmit a first training signal with an (e.g.        omnidirectional or wide-angle) initiator antenna beam to a        responder antenna array configured to receive the first training        signal by successively using different responder antenna beams        of different beam directions.        36. The beamforming device as defined in embodiment 6,        wherein said control unit (31) is configured to control, in the        first training stage, the responder antenna elements (210, 220),        per pair of initiator antenna array (11, 12) and responder        antenna array (21, 22),    -   to transmit the first training signal by successively using        different responder antenna beams of different beam directions        to an initiator antenna array configured to receive the first        training signal with an omnidirectional or wide-angle initiator        antenna beam and    -   subsequently to transmit a first training signal with a (e.g.        omnidirectional or wide-angle) responder antenna beam to an        initiator antenna array configured to receive the first training        signal by successively using different initiator antenna beams        of different beam directions.        37. The beamforming device as defined in embodiment 22,        wherein said data interface (33) is configured to transmit at        least part of said responder selection information and/or to        transmit at least part of said initiator selection information        by use of an additional frame, in particular a control trailer,        and wherein an indicator is used to indicate the use of an        additional frame and/or the amount of information in the        additional frame.        38. A beamforming device for use in a wireless communication        system, wherein said beamforming device is configured to send        feedback by use of SSW feedback frames and/or SSW feedback        fields as disclosed herein, in particular as disclosed in any        one of FIGS. 11-17.        39. The beamforming device as defined in embodiment 38,        said beamforming device being configured for use in a wireless        communication system comprising an initiator and a responder,        said beamforming device comprising:    -   a control unit (31) for controlling the initiator (10) having        one or more initiator antenna arrays (11, 12) each comprising        two or more initiator antenna elements (110, 120) and/or the        responder (20) having one or more responder antenna arrays (21,        22) each comprising two or more responder antenna elements (210,        220), wherein the initiator (10) has at least two initiator        antenna arrays (11, 12) and/or the responder (20) has at least        two responder antenna arrays (21, 22),    -   a processing unit (32) for selecting one or more initiator        antenna beams per initiator antenna array (11, 12) and one or        more responder antenna beams per responder antenna array (21,        22) for use by the initiator (10) and the responder (20) in        communicating with each other.        40. A beamforming method for use in a wireless communication        system, wherein said beamforming method is configured to send        feedback by use of SSW feedback frames and/or SSW feedback        fields as disclosed herein, in particular as disclosed in any        one of FIGS. 11-17.

The invention claimed is:
 1. A beamforming device for use in a wirelesscommunication system comprising an initiator and a responder, saidbeamforming device comprising: circuitry configured to control theinitiator having one or more initiator antenna arrays each comprisingtwo or more initiator antenna elements and/or the responder having oneor more responder antenna arrays each comprising two or more responderantenna elements, wherein the initiator has at least two initiatorantenna arrays and/or the responder has at least two responder antennaarrays, select one or more initiator antenna beams per initiator antennaarray and one or more responder antenna beams per responder antennaarray for use by the initiator and the responder in communicating witheach other, control, in a first training stage, the initiator antennaelements, per pair of initiator antenna array and responder antennaarray, to transmit a first training signal by successively usingdifferent initiator antenna beams of different initiator antenna beamdirections and to receive a first training signal transmitted by theresponder antenna array by successively using different responderantenna beams of different responder antenna beam directions, select asub-set of antenna beam combinations that have been used in the firsttraining stage or are derived from an antenna beam combination used inthe first training stage, for use in a second training stage, wherein atleast some of the antenna beam combinations of said sub-set are selectedby use of first responder quality information indicating a quality ofreception of the first training signals by a respective responderantenna array for the different initiator antenna beams used by arespective initiator antenna array for transmitting the first trainingsignal and first initiator quality information indicating the quality ofreception of the first training signals by the respective initiatorantenna array for the different responder antenna beams used by therespective responder antenna array for transmitting the first trainingsignal in the first training stage, wherein the first responder qualityinformation and the first initiator quality information obtained fordifferent pairs of initiator antenna arrays and responder antenna arraysof the first training stage is used, wherein the circuitry for selectingthe sub-set of antenna beam combinations for use in the second trainingstage includes testing a best obtained beam with crossover variants,wherein during each iteration of testing one or more of random beams forthe initiator antenna arrays and highest quality obtained beams at theresponder are tested, random beams for the responder antenna arrays andhighest quality beams at the initiator are tested, random beams for bothinitiator and responder are tested, and the highest quality obtainedbeams are tested, wherein the random beams are generated such that beamswhich have obtained a high score in the first training phase are morelikely to be chosen, control, in the second training stage, theinitiator antenna elements of the initiator antenna arrays to commonlytransmit a second training signal by successively using differentinitiator antenna beams of different initiator antenna beam directionsaccording to one or more of the selected antenna beam combinations,and/or to receive a second training signal commonly transmitted by theresponder antenna elements of the one or more responder antenna arraysby successively using different responder antenna beams of differentresponder antenna beam directions according to one or more of theselected antenna beam combinations, and select a final antenna beamcombination for use by the initiator and the responder in communicatingwith each other from second responder quality information indicating thequality of reception of the second training signals by the responderantenna arrays for the different initiator antenna beams used by theinitiator antenna arrays for transmitting the second training signaland/or from second initiator quality information indicating the qualityof reception of the second training signals by the initiator antennaarrays for the different responder antenna beams used by the responderantenna arrays for transmitting the second training signal in the secondtraining stage.
 2. The beamforming device as claimed in claim 1, whereinthe circuitry is further configured to control, in the first trainingstage, the initiator antenna elements, per pair of initiator antennaarray and responder antenna array, transmit the first training signal bysuccessively using different initiator antenna beams of different beamdirections to a responder antenna array configured to receive the firsttraining signal with an omnidirectional or wide-angle responder antennabeam, and receive a first training signal transmitted by a responderantenna array by successively using different responder antenna beams ofdifferent beam directions, wherein the initiator antenna array isconfigured to receive the first training signal with an omnidirectionalor wide-angle initiator antenna beam.
 3. The beamforming device asclaimed in claim 1, wherein the circuitry is further configured tocontrol, in the first training stage, the initiator antenna elements,per pair of initiator antenna array and responder antenna array,transmit the first training signal by successively using differentinitiator antenna beams of different initiator antenna beam directionsto a responder antenna configured to receive the first training signalwith a directed responder antenna beam, wherein the successivetransmission of the first training signal with different initiatorantenna beams of different initiator antenna beam directions is repeatedmultiple times, wherein in each iteration one or more different directedresponder antenna beams are used for reception, and receive the firsttraining signal transmitted from a responder antenna array bysuccessively using different responder antenna beams of differentresponder antenna beam directions, wherein the initiator antenna arrayis configured to receive the first training signal with a directedinitiator antenna beam, wherein the successive transmission of the firsttraining signal with different responder antenna beams of differentresponder antenna beam directions is repeated multiple times, wherein ineach iteration one or more different directed initiator antenna beamsare used for reception.
 4. The beamforming device as claimed in claim 1,wherein the circuitry is further configured to control, in a firsttraining stage, the initiator antenna elements, per two or more pairs ofinitiator antenna arrays and responder antenna arrays, transmit a firsttraining signal successively with different initiator antenna beams ofdifferent initiator antenna beam directions to a responder antenna,wherein the initiator antenna elements of different initiator antennaarrays simultaneously transmit orthogonal first training signals and/orover different polarization, and receive a first training signaltransmitted from a responder antenna array successively with differentresponder antenna beams of different responder antenna beam directions,wherein the responder antenna elements of different responder antennaarrays simultaneously transmit orthogonal first training signals and/orover different polarization.
 5. The beamforming device as claimed inclaim 1, wherein the circuitry is further configured to control in afirst phase of the first and/or second training stage, the initiatorantenna elements to transmit the training signal by successively usingdifferent initiator antenna beams having a first beam width and/or theresponder antenna elements to receive the training signal bysuccessively using different responder antenna beams having a first beamwidth, select at least part of the antenna beam combinations of thesub-set of antenna beam combinations for use in a second phase of a sametraining stage based on the initiator antenna beam direction of theinitiator antenna beam selected based on the responder qualityinformation, and control in the second phase, the initiator antennaelements to transmit the training signal by successively using differentinitiator antenna beams having a second beam width different from thefirst beam width used in the first phase and having an initiator antennabeam direction identical or similar as the initiator antenna beamdirection of the initiator antenna beam providing a best first initiatorquality information in the first phase and/or the responder antennaelements to receive the training signal by successively using differentresponder antenna beams having a second beam width different from thefirst beam width used in the first phase and having a responder antennabeam direction identical or similar as the responder antenna beamdirection of the responder antenna beam providing the best firstresponder quality information in the first phase.
 6. The beamformingdevice as claimed in claim 1, wherein the circuitry is configured toselect, per pair of initiator antenna array and responder antenna array,combinations of initiator antenna beams and responder antenna beams tobe used in the first training stage by use of a genetic or evolutionarysearch algorithm.
 7. The beamforming device as claimed in claim 1,wherein the first initiator quality information and the first responderquality information is information indicating signal to noise ratio,signal to noise-and-interference ratio, receive signal strengthindication, an estimated capacity, received electric or magnetic fieldstrength or delay spread per pair of initiator antenna array andresponder antenna array and per antenna beam.
 8. The beamforming deviceas claimed in claim 1, wherein the second initiator quality informationand/or the second responder quality information is informationindicating the estimated capacity, sum of singular values, or conditionnumber of a channel matrix per antenna beam combination.
 9. Thebeamforming device as claimed in claim 1, wherein the circuitry isconfigured to determine an antenna score per initiator antenna array andper responder antenna array based on the first initiator qualityinformation and the first responder quality information of the antennabeam combinations used in the first training stage, said antenna scoresincluding a score value per antenna beam used in the first trainingstage, and to use the determined antenna scores for selecting thesub-set of antenna beam combinations for use in the second trainingstage.
 10. The beamforming device as claimed in claim 9, wherein thecircuitry is configured to calculate an overall score for differentcomplete antenna beam combinations of antenna beams from the differentinitiator antenna arrays and the different responder antenna arrays as aproduct, sum, average or linear combination of the antenna scores and touse the calculated overall scores for selecting the sub-set of antennabeam combinations for use in the second training stage.
 11. Thebeamforming device as claimed in claim 9, wherein the circuitry isconfigured to sort the antenna scores per initiator antenna array andper responder antenna array and to select the sub-set of antenna beamcombinations for use in the second training stage based on the sortedantenna scores by selecting the antenna beam combinations having a bestscore and/or by use of a probability distribution determined from theantenna scores, and to select antenna beam combinations for use in thesecond training stage, in which one or more antenna beams are replacedby one or more of its nearest neighbors in the sorted antenna scores orby one or more antenna beams, randomly selected according to the alreadydetermined probability distribution function.
 12. The beamforming deviceas claimed in claim 1, wherein the circuitry is configured to calculatean overall score for different complete antenna beam combinations ofantenna beams from the different initiator antenna arrays and thedifferent responder antenna arrays based on the first initiator qualityinformation and the first responder quality information of the antennabeam combinations used in the first training stage and to use thecalculated overall scores for selecting the sub-set of antenna beamcombinations for use in the second training stage.
 13. The beamformingdevice as claimed in claim 1, wherein the circuitry is configured toselect the sub-set of antenna beam combinations for use in the secondtraining stage by use of a genetic or evolutionary search algorithm. 14.The beamforming device as claimed in claim 13, wherein the circuitry isconfigured to select part of the antenna beam combinations of thesub-set for use in the second training stage randomly, in particular byuse of a uniform or non-uniform probability distribution.
 15. Thebeamforming device as claimed in claim 13, wherein the circuitry isconfigured to determine, in each iteration in the second training stage,the second responder quality information and/or the second initiatorquality information, to determine an overall score for an overallantenna beam combination used in said iteration and to compare thedetermined overall score with the overall score of the previousiterations.
 16. The beamforming device as claimed in claim 15, whereinthe circuitry is configured to set, in each iteration, the overallantenna beam combination having the best overall score up to saiditeration, as preliminary best antenna beam combination and to selectthe overall antenna beam combination to be used in a next iterationbased on the preliminary best antenna beam combination.
 17. Thebeamforming device as claimed in claim 1, wherein the circuitry isconfigured to use the beam direction of the initiator antenna beamand/or the responder antenna beam providing the best initiator qualityinformation and the best responder quality information for selectingantenna beam combinations used subsequently in the first training stageand/or the second training stage.
 18. The beamforming device as claimedin claim 1, wherein the circuitry is configured to repeat the firstand/or second training stage if a signal level of the communicationdecreases or the quality of the communication decreases or a trigger isissued or time-out is reached, wherein one or more final antenna beamcombinations used earlier are used as a start for selecting antenna beamcombinations in the first and/or second training phase.
 19. Thebeamforming device as claimed in claim 1, wherein the circuitry isconfigured to stop the first and/or second training stage if a time-outis reached or a predetermined number of antenna combinations have beentested or the improvement with respect to past iterations and/or withrespect to best obtained metric decreases below a predeterminedthreshold or the obtained metric exceeds an absolute upper threshold.20. The beamforming device as claimed in claim 1, further comprising adata interface for at least one of receiving the first responder qualityinformation receiving the second responder quality information from theresponder, transmitting the first initiator quality information,transmitting the second initiator quality information to the responder,transmitting the first responder quality information, transmitting thesecond responder quality information to the initiator, receiving thefirst initiator quality information, and receiving the second initiatorquality information from the initiator.
 21. The beamforming device asclaimed in claim 20, wherein said data interface is configured totransmit at least one of responder selection information indicating theresponder antenna beams of the selected sub-set of antenna beamcombinations to the responder for use in the second training stage andinitiator selection information indicating the initiator antenna beamsof the selected sub-set of antenna beam combinations to the initiatorfor use in the second training stage.
 22. The beamforming device asclaimed in claim 21, wherein said data interface is configured totransmit at least part of said responder selection information and/or totransmit at least part of said initiator selection information by use ofan additional frame, in particular a control trailer, and wherein anindicator is used to indicate the use of an additional frame and/or anamount of information in the additional frame.
 23. The beamformingdevice as claimed in claim 20, wherein the circuitry is configured tocontrol at least one of said initiator antenna elements to transmitinitiator quality information and/or responder selection informationwithin or along with first, second training signals transmitted to theresponder, said responder antenna elements to transmit responder qualityinformation, initiator selection information within or along with first,and second training signals transmitted to the initiator.
 24. Thebeamforming device as claimed in claim 1, wherein the circuitry isconfigured to calculate one or more of the first responder qualityinformation, the second responder quality information, the firstinitiator quality information and the second initiator qualityinformation.
 25. The beamforming device as claimed in claim 1, whereinthe circuitry is configured to control, in the first training stage, theinitiator antenna elements, per pair of initiator antenna array andresponder antenna array, transmit the first training signal bysuccessively using different initiator antenna beams of different beamdirections to a responder antenna array configured to receive the firsttraining signal with an omnidirectional or wide-angle responder antennabeam, and subsequently transmit a first training signal with aninitiator antenna beam to a responder antenna array configured toreceive the first training signal by successively using differentresponder antenna beams of different beam directions.
 26. A beamformingdevice for use in a wireless communication system comprising aninitiator and a responder, said beamforming device comprising: circuitryconfigured to control the initiator having one or more initiator antennaarrays each comprising two or more initiator antenna elements and/or theresponder having one or more responder antenna arrays each comprisingtwo or more responder antenna elements, wherein the initiator has atleast two initiator antenna arrays and/or the responder has at least tworesponder antenna arrays, select one or more initiator antenna beams perinitiator antenna array and one or more responder antenna beams perresponder antenna array for use by the initiator and the responder incommunicating with each other, control, in a first training stage, theresponder antenna elements, per pair of initiator antenna array andresponder antenna array to receive a first training signal transmittedby the initiator antenna array by successively using different initiatorantenna beams of different initiator antenna beam directions and totransmit a first training signal by successively using differentresponder antenna beams of different responder antenna beam directions,select a sub-set of the antenna beam combinations that have been used inthe first training stage or are derived from the antenna beamcombination used in the first training stage, for use in a secondtraining stage, wherein at least some of the antenna beam combinationsof said sub-set are selected by use of first responder qualityinformation indicating the quality of reception of the first trainingsignals by the respective responder antenna array for the differentinitiator antenna beams used by the respective initiator antenna arrayfor transmitting the first training signal and first initiator qualityinformation indicating the quality of reception of the first trainingsignals by the respective initiator antenna array for the differentresponder antenna beams used by the respective responder antenna arrayfor transmitting the first training signal in the first training stage,wherein the first responder quality information and the first initiatorquality information obtained for the different pairs of initiatorantenna arrays and responder antenna arrays of the first training stageis used, wherein the circuitry for selecting the sub-set of antenna beamcombinations for use in the second training stage includes testing abest obtained beam with crossover variants, wherein during eachiteration of testing one or more of random beams for the initiatorantenna arrays and highest quality obtained beams at the responder aretested, random beams for the responder antenna arrays and highestquality beams at the initiator are tested, random beams for bothinitiator and responder are tested, and the highest quality obtainedbeams are tested, wherein the random beams are generated such that beamswhich have obtained a high score in the first training phase are morelikely to be chosen, control, in the second training stage, theresponder antenna elements of the responder antenna arrays to receive asecond training signal commonly transmitted by the initiator antennaelements of the one or more initiator antenna arrays by successivelyusing different initiator antenna beams of different initiator antennabeam directions according to one or more of the selected antenna beamcombinations and/or to commonly transmit a second training signal bysuccessively using different initiator antenna beams of differentinitiator antenna beam directions according to one or more of theselected antenna beam combinations, and select a final antenna beamcombination for use by the initiator and the responder in communicatingwith each other from second responder quality information indicating thequality of reception of the second training signals by the responderantenna arrays for the different initiator antenna beams used by theinitiator antenna arrays for transmitting the second training signaland/or from second initiator quality information indicating the qualityof reception of the second training signals by the initiator antennaarrays for the different responder antenna beams used by the responderantenna arrays for transmitting the second training signal in the secondtraining stage.
 27. A beamforming method for use in a wirelesscommunication system comprising an initiator and a responder, theinitiator having one or more initiator antenna arrays each comprisingtwo or more initiator antenna elements and/or the responder having oneor more responder antenna arrays each comprising two or more responderantenna elements, wherein the initiator has at least two initiatorantenna arrays and/or the responder has at least two responder antennaarrays, said beamforming method being configured for selecting one ormore initiator antenna beams per initiator antenna array and one or moreresponder antenna beams per responder antenna array for use by theinitiator and the responder in communicating with each other, saidbeamforming method comprising: controlling, in a first training stage,the initiator antenna elements, per pair of initiator antenna array andresponder antenna array, to transmit a first training signal bysuccessively using different initiator antenna beams of differentinitiator antenna beam directions and to receive a first training signaltransmitted by the responder antenna array by successively usingdifferent responder antenna beams of different responder antenna beamdirections, selecting a sub-set of the antenna beam combinations thathave been used in the first training stage or are derived from theantenna beam combination used in the first training stage, for use in asecond training stage, wherein at least some of the antenna beamcombinations of said sub-set are selected by use of first responderquality information indicating the quality of reception of the firsttraining signals by the respective responder antenna array for thedifferent initiator antenna beams used by the respective initiatorantenna array for transmitting the first training signal and firstinitiator quality information indicating the quality of reception of thefirst training signals by the respective initiator antenna array for thedifferent responder antenna beams used by the respective responderantenna array for transmitting the first training signal in the firsttraining stage, wherein the first responder quality information and thefirst initiator quality information obtained for the different pairs ofinitiator antenna arrays and responder antenna arrays of the firsttraining stage is used, wherein selecting the sub-set of antenna beamcombinations for use in the second training stage includes testing abest obtained beam with crossover variants, wherein during eachiteration of testing one or more of random beams for the initiatorantenna arrays and highest quality obtained beams at the responder aretested, random beams for the responder antenna arrays and highestquality beams at the initiator are tested, random beams for bothinitiator and responder are tested, and the highest quality obtainedbeams are tested, wherein the random beams are generated such that beamswhich have obtained a high score in the first training phase are morelikely to be chosen, controlling, in the second training stage, theinitiator antenna elements of the initiator antenna arrays to commonlytransmit a second training signal by successively using differentinitiator antenna beams of different initiator antenna beam directionsaccording to the selected antenna beam combinations, and/or to receive asecond training signal commonly transmitted by the responder antennaelements of the one or more responder antenna arrays by successivelyusing different responder antenna beams of different responder antennabeam directions according to the selected antenna beam combinations, andselecting a final antenna beam combination for use by the initiator andthe responder in communicating with each other from second responderquality information indicating the quality of reception of the secondtraining signals by the responder antenna arrays for the differentinitiator antenna beams used by the initiator antenna arrays fortransmitting the second training signal and/or from second initiatorquality information indicating the quality of reception of the secondtraining signals by the initiator antenna arrays for the differentresponder antenna beams used by the responder antenna arrays fortransmitting the second training signal in the second training stage.28. A beamforming method for use in a wireless communication systemcomprising an initiator and a responder, the initiator having one ormore initiator antenna arrays each comprising two or more initiatorantenna elements and/or the responder having one or more responderantenna arrays each comprising two or more responder antenna elements,wherein the initiator has at least two initiator antenna arrays and/orthe responder has at least two responder antenna arrays, saidbeamforming method being configured for selecting one or more initiatorantenna beams per initiator antenna array and one or more responderantenna beams per responder antenna array for use by the initiator andthe responder in communicating with each other, said beamforming methodcomprising: controlling, in a first training stage, the responderantenna elements, per pair of initiator antenna array and responderantenna array to receive a first training signal transmitted by theinitiator antenna array by successively using different initiatorantenna beams of different initiator antenna beam directions and totransmit a first training signal by successively using differentresponder antenna beams of different responder antenna beam directions,selecting a sub-set of the antenna beam combinations that have been usedin the first training stage or are derived from the antenna beamcombination used in the first training stage, for use in a secondtraining stage, wherein at least some of the antenna beam combinationsof said sub-set are selected by use of first responder qualityinformation indicating the quality of reception of the first trainingsignals by the respective responder antenna array for the differentinitiator antenna beams used by the respective initiator antenna arrayfor transmitting the first training signal and first initiator qualityinformation indicating the quality of reception of the first trainingsignals by the respective initiator antenna array for the differentresponder antenna beams used by the respective responder antenna arrayfor transmitting the first training signal in the first training stage,wherein the first responder quality information and the first initiatorquality information obtained for the different pairs of initiatorantenna arrays and responder antenna arrays of the first training stageis used, wherein selecting the sub-set of antenna beam combinations foruse in the second training stage includes testing a best obtained beamwith crossover variants, wherein during each iteration of testing one ormore of random beams for the initiator antenna arrays and highestquality obtained beams at the responder are tested, random beams for theresponder antenna arrays and highest quality beams at the initiator aretested, random beams for both initiator and responder are tested, andthe highest quality obtained beams are tested, wherein the random beamsare generated such that beams which have obtained a high score in thefirst training phase are more likely to be chosen, controlling, in thesecond training stage, the responder antenna elements of the responderantenna arrays to receive a second training signal commonly transmittedby the initiator antenna elements of the one or more initiator antennaarrays by successively using different initiator antenna beams ofdifferent initiator antenna beam directions according to the selectedantenna beam combinations and/or to commonly transmit a second trainingsignal by successively using different initiator antenna beams ofdifferent initiator antenna beam directions according to the selectedantenna beam combinations, and selecting a final antenna beamcombination for use by the initiator and the responder in communicatingwith each other from second responder quality information indicating thequality of reception of the second training signals by the responderantenna arrays for the different initiator antenna beams used by theinitiator antenna arrays for transmitting the second training signaland/or from second initiator quality information indicating the qualityof reception of the second training signals by the initiator antennaarrays for the different responder antenna beams used by the responderantenna arrays for transmitting the second training signal in the secondtraining stage.
 29. A communication device for communicating withanother communication device in a wireless communication system, saidcommunication device comprising: one or more antenna arrays eachcomprising two or more antenna element, and a beamforming device asclaimed in claim
 1. 30. A communication system including a beamformingdevice as claimed in claim 1 and two or more communication devices, eachhaving at least one antenna array each comprising two or more antennaelement, wherein at least one communication device has at least twoantenna arrays.
 31. A non-transitory computer-readable recording mediumthat stores therein a computer program product, which, when executed bya processor, causes a beamforming method according to claim 27 to beperformed.
 32. A beamforming device for use in a wireless communicationsystem, wherein said beamforming device is configured to transmit atleast part of responder selection information and/or initiator selectioninformation by use of an additional frame, in particular a controltrailer, wherein an indicator is used to indicate the use of anadditional frame and/or the amount of information in the additionalframe, wherein said responder selection information indicates theresponder antenna beams of a selected sub-set of antenna beamcombinations to a responder for use in a second training stage and/orsaid initiator selection information indicates the initiator antennabeams of a selected sub-set of antenna beam combinations to an initiatorfor use in a second training stage, wherein the selected sub-set ofantenna beam combinations for use in the second training stage includestesting a best obtained beam with crossover variants, wherein duringeach iteration of testing one or more of random beams for the initiatorantenna arrays and highest quality obtained beams at the responder aretested, random beams for the responder antenna arrays and highestquality beams at the initiator are tested, random beams for bothinitiator and responder are tested, and the highest quality obtainedbeams are tested, wherein the random beams are generated such that beamswhich have obtained a high score in a first training phase are morelikely to be chosen.